David Hume's «Inquiry Concerning Human Understanding».

A shorter version of this article can be found here.

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a) Exposure

In its Research on Human Understanding, David Hume radicalizes the empiricist approach of John Locke. Recall that Descartes I had started from the “Cogito”, from our thought (the fact that we think is a sure thing. Later we will look for the reasons why we should trust our thought, in order to lay the firm foundations of our knowledge). And let us remember that Locke had started from “ideas”, understood in the Greek sense of the word: “that which is seen”. That is to say that he started from what was perceived by the senses, external or internal. Hume, more radical, will distinguish between “impression” and “idea”, since it is not the same thing what we are seeing or feeling -the impression- than what we have seen or felt -the idea, the seen or the felt-, as it is not the same thing a toothache that “I feel” than a toothache that I have felt, I much prefer the latter, that is to say, the merely remembered.

Having made this distinction, it is clear that the basis of his analysis of knowledge will not be ideas, but, more radically, impressions. I now look and have an impression, I close my eyes and that impression ceases. The idea remains, the thing seen, the memory of the impression, and the confidence in an existence independent of me, for I trust that, if I open my eyes again, the same thing will appear before them again. But, sensu stricto, during the time I had my eyes closed there was only the mere confidence that this perception would be repeated, a confidence based on the mere habit that other times it has happened like that, but not based on true reasoning. 

The following analysis will show us that, in fact, true knowledge is not possible in questions of existence. Let us anticipate that, whenever we speak here of an idea, it will always be, of course, of a particular idea, for already in his earlier Treatise on Human Nature, In Hume, as in his predecessor Berkeley, there is no trace of the notion of abstraction, for reasons that, for what I have said, must already be obvious.  

His analysis of ideas is based on the observation that the ideas we have are either the memory of a past impression, or are formed from other ideas by association, either association by resemblance or by contiguity, or by causality. There is association by resemblance of ideas, because it is known that some ideas suggest to us others with which they bear a certain similarity, as the sight of a portrait brings me by association the idea of the person portrayed. There is association by contiguity in time or place, because, when two ideas are contiguous, one of them suggests the other to us (as in the neighborhood: the idea of an apartment in a building suggests the idea of the “contiguous apartment”); and there is also association by causality, or association of one idea to another as its cause, that is, as if there were a certain necessary connection between them (the idea of a wound inflicted on me is necessarily followed by the idea of pain, so we say that the wound is the cause of the pain). It is this last type of association of ideas that most interests Hume, because he sees in it the main source of our chimerical ideas, and hence the main source of error in our knowledge.

And this is so, because, when we have often perceived a certain temporal contiguity between two ideas - it is always first the wound and then the pain - we end up imagining that there is a necessary connection between the two, as if the first must necessarily be followed by the second, a necessity that we have never demonstrated, but simply a temporal contiguity to which we have become accustomed. And we express this then by saying that the first is cause and the second is effect. 

But this is false knowledge because it takes for granted what is only supposed since we do not perceive such a connection - we have no impression of it - nor do we deduce it by any reasoning. Whenever we eat bread (food) we are next comforted, and whenever we see brightness and heat (flame) we next see that a paper placed nearby is charred, its color turning black, and we then say that the former is the cause of the latter, as if the latter necessarily followed from the former. But there is no reasoning by which we can conclude such a necessity, nor can there ever be. We can never find any reason why those perceptions which we call bread - a color, a taste, and even a pleasant smell if it is fresh - should necessarily entail those other internal perceptions of feeling comforted, restored, satiated, after consuming it; or why that brightness and warmth which we call fire should necessarily entail the perception of that blackening of a piece of paper which we call carbonization. We are accustomed to it, simply, and that is the only basis of our assurance that it will continue to happen in the future: pure habituation and not the perception or demonstration of a necessary connection between what we call cause and what we call effect. 

Causality is thus unmasked as mere belief: mere habituation to a certain temporal contiguity between impressions, on which to base a mere confidence that future experiences will be like past ones. This is how causality is at the basis of our beliefs of existence. I see a letter, but I do not see its author, but I infer, nevertheless, that an author must “exist,” for someone must have been the cause of that letter being written. Thus, a new idea, that of author, was created in connection with an idea I already had, the idea of letter, calling then “idea” or “seen” the author, when all I see is the letter.

In particular, I form the idea that something external “exists” that causes the impressions that occur in me, although, strictly speaking, I only have those impressions. By “existence” I understand its independence from myself, even when I no longer perceive anything because I have closed my eyes, as I said at the beginning. I have this existence as “perceived” - although in fact I do not perceive it - as a conviction that when I open my eyes again I will again receive those same impressions. This conviction, as I have said, is not true knowledge, for it is not based on reasoning but only on habituation: on the mere confidence that it will happen in the future as it has happened in the past, confidence based on causality - on something, then, that we have already unmasked as chimerical - for I think that the impressions have been caused by something external to me.

In fact, Hume's radicalism leads him to unmask as chimerical the very concept of the “I”, since we have no perception of it. He reduces it to a collection of perceptions, of which he says that we would not have the notion of “I” if it were not for the memory with which we are able to keep recollection of past perceptions. But remembering is not the same as perceiving, so the “I” enters into his catalog of chimerical ideas, added to the chimerical idea of the external world, which is followed, of course, by the chimerical idea of God. 

Now that we understand that causality is mere belief, or trust, and not properly knowledge, what can we say of freedom, of that concept that allows us to speak of moral responsibility, which is at the basis of the very science of ethics? When we perceive that one stone strikes another, we say that the movement of the latter has been caused by the movement of the former, although such a necessary connection has not been demonstrated (if such a demonstration were given, we would not have to have seen it many times but only once would have sufficed, for when true knowledge is given, when we are presented with a reasoning that we recognize as true, for example, it is enough for us to have seen it only once; but habituation - for it is only that, habituation - demands having seen it many times, because in reality it is not true knowledge). However, we do not therefore say that the movement of the second stone is free, but that it necessarily follows from the movement of the first. But when it is my will that orders the movement of a body, of my own body, so that it follows the order of my will, we no longer speak of necessity, but we speak of a free act. Why is this act free and not the first, if it is the same thing, of pure habituation to the fact that the first - what we call cause or moral responsible - is always followed by the second, the movement of a stone or of my own body? Freedom is therefore a mere illusion, and there is therefore no reason to speak of moral responsibility. In short, it is the same as always: to find a culprit. 

Having stated his gnoseological position, Hume says that he is not in favor of a Pyrrhonian skepticism either, for which nothing at all signifies existence, but mere illusion from what is really seen by our senses, without any reason for us to trust such an illusion. Hume says that this radical skeptic is unbeatable in the academy, that is, in philosophical debate. But when he goes out into life itself, the skeptic is defeated by those who are not skeptics, but rather rely on all that they take for knowledge. Indeed, on stumbling upon a bonfire the skeptic finds no reason to recoil from it, but in fact recoils as if he had knowledge that such a bonfire exists, independent of him. This is why Hume advocates a reasonable and beneficial skepticism, a moderate skepticism: it would be a matter, yes, of admitting causality and therefore existence, but not as true knowledge, since it is not, but simply as belief or confidence based on custom. Since we admit it for practical rather than gnoseological reasons, we will not give ourselves unwisely to the fire, nor will we make any immolation folly by being skeptical. 

And furthermore we will cultivate the sciences, yes, but without speaking of necessary connections where we do not see them, nor do we demonstrate them, but of repeated temporal contiguity until now, understanding that no more than that are the universal laws, such as that which says that iron is dilated by heat.

Such moderate or beneficial skepticism will thus leave the sciences in their rightful place, reduced to the science of what makes sense, and unmasking as sophistry and deception other branches of knowledge of which he will give an account below. 

Hume distinguishes between two kinds of knowledge: one is that which relates ideas, and this proceeds by means of reasoning articulated in demonstrations; and the other is that which refers to matters of fact or existence, which does not proceed by demonstrations but only with moral certainty, and which cannot be called knowledge because it is based on belief.  

The first category includes geometry and arithmetic, i.e. mathematics. These are true knowledge because in these disciplines there are demonstrations that relate ideas in an irrefutable way. However, he shows his skepticism about the newly founded infinitesimal calculus, which was developing in his time: healthy skepticism, inherited from George Berkeley, and I say healthy because, as a mathematician, I can assure you that David Hume was not wrong in this, since the infinitesimal calculus was only founded, articulated in a clear and distinct way, in the following nineteenth century. (His immediate philosophical predecessor, the Anglican bishop George Berkeley, said that mathematicians make a truth out of two lies, and he was not wrong, nor was he unfortunate in saying so, since his attacks, and others that followed by mathematicians themselves, served as a stimulus for the formalization of calculus in the following century, which required the formalization of all mathematics, and for it, the creation of formal logic, in which was born the theory of machines that has led to today's computers).

They follow the theoretical sciences or sciences of demonstration - geometry and arithmetic - the experimental sciences, the so-called natural sciences, i.e. those which deal with questions of fact and existence. They do not proceed falsely, as long as they understand their laws for what they are, as a simple record of the repetition so far of a certain contiguity of facts. Their expression as a law of nature is to be understood only as an expression of our confidence that it will occur in the future as it has hitherto occurred in the past, but in no way as an expression of a necessary connection between facts: in saying “when iron is heated, its dilatation follows,” we shall not understand that there is a necessary connection between the two facts, for we neither perceive it nor can ever perceive it, but only that we are confident that it will occur in the future as it has hitherto occurred. 

And we come to the other kinds of knowledge, to those investigations about ideas that have not come to us through the senses, nor are they associated with ideas perceived by the senses. Of these misnamed ideas, for no one has seen them, David Hume says: “When we have a suspicion that a philosophical term is employed without any meaning or idea (as happens even too frequently) we need but inquire: from what impression is this supposed idea derived? And if it is impossible to assign any to it, it will serve to confirm our suspicion” What David Hume thinks about such supposed knowledge, particularly of metaphysics, is well captured in the concluding words of his work:  

“When we go through libraries, persuaded of these principles, what havoc will we not wreak! If we take in our hands a volume of theology or scholastic metaphysics, for example, let us ask ourselves: does it contain any abstract reasoning about quantity and number? No. Does it contain any experimental reasoning about matters of fact and experience? No. Throw it then into the flames, for it can contain nothing but sophistry and delusion.”  

 b) Texts

I. On the origin of ideas

There is a great difference between the perceptions of the mind when a man feels the pain of an excessive heat or the pleasure of a moderate one, and his perceptions when he later recalls in his memory this sensation.... 

These less strong and vivid [perceptions of the mind] are commonly called thoughts or ideas. The other species...we will call impressions. 

All the materials of thought are derived from our external or internal sensibility; the mind and the will have only the task of mixing and composing them.

When we have a suspicion that a philosophical term is used without any meaning or idea (as happens all too frequently) we need only ask ourselves: from what impression is this supposed idea derived? And if it is impossible to assign any to it, this will serve to confirm our suspicion. 

II. On the association of ideas

It seems to me that there are only three principles of connection between ideas, namely: similarity, contiguity in time or place, and cause or effect....

A painting naturally leads our thoughts to the original (similarity); the mention of an apartment in a building naturally introduces an inquiry or a discourse concerning the others (contiguity); and if we think of a wound, we can hardly avoid reflecting on the pain that follows (cause and effect). 

III. Skeptical doubts about the operations of the understanding.

All objects which fall under human reason or investigation may be naturally divided into two classes, namely, relations of ideas and matters of fact. Of the first class are the sciences of geometry, algebra, and arithmetic, and, in short, every statement that is intuitively or demonstratively true.....

All reasoning on matters of fact seems to be founded on the relation of cause and effect. Only by means of this relation can we go beyond the evidence of our memory and our senses. If you were to ask a man why he believes a matter of fact which is not at present evident, as, for instance, that his friend is in the country, or in France, he would give a reason, and this reason would be some additional fact, such as a letter of his which he has received or the knowledge of his former resolutions and promises A man finding a watch or any other machine on a desert island, would conclude that there was once a man on it. All our reasonings about facts are of the same nature. In them there is constantly assumed to be a connection between the present fact and the one inferred from it. If there were nothing to link them, the inference would be utterly precarious. Hearing in the dark an articulate voice and rational speech assures us of the presence of some person. Because these are effects of man's constitution and structure closely connected with them. If we dissect the rest of reasonings of this nature, we would find that they are founded on the relation of cause and effect, whether this relation be close or remote, direct or collateral. Heat and light are collateral effects of fire, and one may be correctly inferred from the other.

 If, therefore, we are to be satisfied as to the nature of this evidence that assures us of matters of fact, we must investigate how we arrive at the knowledge of cause and effect.  

 I will venture to assert, as a general proposition which admits of no exception, that knowledge of this relation is in no case obtained by a priori reasoning; but is born entirely of experience when we find that any particular objects are constantly conjunct with each other. 

Causes and effects are not discovered by reason but by experience...for no one imagines that the explosion of gunpowder or the attraction of the magnet can ever be discovered by a priori arguments....Who will claim to be able to give the ultimate reason why milk or bread are suitable food for man, and not for a lion or a tiger? 

The mind can never find the effect in the supposed cause, even by the most minute examination and scrutiny; for the effect is entirely different from the cause, and therefore can never be discovered in the latter. The motion of the second billiard ball is an entirely different event from the motion of the first, and there is nothing in the one to suggest the slightest indication of the other..... 

No a priori reasoning will ever be able to substantiate it.

It is conceded that the greatest effort of human reason is to reduce the principles producing natural phenomena to a greater simplicity and to resolve the multiple particular effects into a few general causes by means of analytical reasoning, experience and observation. But as to the causes of these general causes, we would in vain attempt their discovery. Elasticity, gravity, cohesion of parts, communication of motion by impulse; these are, probably, the last causes and principles which we shall ever discover in nature, and we may consider ourselves happy enough, if, by careful investigation and reasoning, we can trace particular phenomena back to these general principles, or even near them. The most perfect natural philosophy only pushes our ignorance a little farther away.

Thus, a law of motion, discovered by experience, that the momentum or force of any moving body is in compound ratio or proportion to its mass and its velocity... The very discovery of the law is due merely to experience and all the abstract reasoning of the world could never advance us a step further towards its knowledge....

Our senses inform us of the color, weight and consistency of bread; but neither the senses nor reason can ever inform us of those qualities that make it suitable for the nutrition and sustenance of a human body....

It is conceded by all that no connection is known between sensible qualities and secret powers..... As regards past experience, it may be conceded that it furnishes direct and certain information only of those objects, and for that precise period of time, which fall under its cognizance; but why this experience should extend to future times and to other objects which, for all we know, may be only in appearance similar, is the main question on which I should like to insist. The bread, which I formerly ate, nourished me; that is, a body of such and such sensible qualities was, at that time, endowed with such and such secret powers. But does it follow that another bread, at another time must likewise nourish me, and that similar sensible qualities must always be accompanied by similar secret powers? The consequence seems by no means necessary. At least, it must be recognized that there is here a consequence that the mind draws, that a certain step is taken, a process of thought and an inference that needs to be explained. These two propositions are far from being the same: I have found that such an object has always been accompanied by such an effect, and I foresee that other objects, in appearance similar, will be accompanied by similar effects. I will grant, if you please, that one proposition may properly be inferred from the other. Indeed, I know that it is always inferred. But if you insist that the inference is made by a chain of reasonings, I will ask you to reproduce this reasoning. 

All reasonings may be divided into two classes, namely, into demonstrative reasonings, or concerning relations of ideas, and into moral reasonings, or concerning questions of fact and existence. That there are no demonstrative arguments in this case seems evident. 

All arguments about existence are founded on the relation of cause and effect, our knowledge of this relation is derived entirely from experience, and all our experimental conclusions proceed on the assumption that the future will be in conformity with the past.... 

If it were said that from a number of uniform experiments we infer a connection between sensible qualities and secret powers, I must confess that this seems to me to involve the same difficulty already expressed in other terms. The question again arises, on what process of argumentation is this inference founded?....

When a man says “I have found in all past cases such sensible qualities coupled with such secret powers”; and when he says “similar sensible qualities will always be coupled with similar secret powers” ...to say that this is experimental is to make a plea of principle. For all inferences from experience assume, as their foundation, that the future will resemble the past....

When a child has experienced the sensation of pain on touching the flame of a candle, he will be careful not to put his hand near a candle; and he will expect a similar effect from a cause which is similar in its appearance and in its sensible qualities. If you assert, therefore, that the child's understanding is led to this conclusion by a process of argumentation or reasoning, I may rightfully require of you the reproduction of this argument.....

IV. Skeptical solution to these doubts

Yet, with all his experience, [a person] has acquired no idea or knowledge of the secret power by which one object produces the other; nor is it by any reasoning that he is compelled to make this inference. 

This principle is custom or habit. For wherever the repetition of any particular act or operation produces a propensity to renew this same act or operation...Heat and flame, for instance, or weight and solidity. We are determined only by habit to expect the one on the occasion of the appearance of the other...All inferences, therefore, are effects of habit, not of reasoning. 

Custom is the principle by which this correspondence, so necessary for the subsistence of our species, has been produced.

V. On probability

Although there is no such thing in the world as chance, our ignorance of the real cause of any event has the same influence on the understanding, giving rise to a similar sort of belief or opinion

Determined as we are by habit to transfer the past to the future in all our inferences, where the past has been completely regular and uniform we expect the event with greater certainty.

VI. On the idea of necessary connection

The great advantage of the mathematical sciences over the moral sciences lies in the fact that the ideas of the former are always clear and definite... The isosceles and the scalene are differentiated by more exact limits than vice and virtue.... 

The chief obstacle, therefore, to our progress in the moral or metaphysical sciences is the obscurity of ideas and the ambiguity of terms...3 There are no ideas, of those given in metaphysics, more obscure and uncertain than those of power, force, energy or necessary connection....

Our ideas are nothing but copies of our impressions, or in other words, it is impossible for us to think anything that we have not previously felt by means of our external or internal senses... Complex ideas can, perhaps, be well known by definition, which is nothing but an enumeration of those parts or simple ideas that compose them. 

When we look around us at external objects and consider the operation of causes, we are never able, from a single instance, to discover any power or necessary connection... We only find that, indeed, in fact, the one follows the other. ... The mind feels no feeling or internal impression from this succession of objects. Consequently, there is... nothing to suggest the idea of power or necessary connection. 

But if the mind could discover the power or energy of a cause, we could foresee the effect even without experience....

We know that, in fact, heat constantly accompanies the flame; but what is the connection between them is something that we cannot even conjecture or imagine....

The movement of our body follows the command of our will. From this we are aware that one event constantly follows another, without instructing us in the secret connection that binds them and makes them inseparable...

We ignore, it is true, the way bodies operate with each other. Their force or energy is utterly incomprehensible. But are we not equally ignorant of the manner or force by which a mind, even the supreme mind, operates upon itself or upon a body? ... All we know is our profound ignorance in both cases.....

So that, in short, no instance of connection that is conceivable to us manifests itself in the whole of nature. All events seem completely detached and separate. One event follows another, but we can never observe any link between them.... 

But when a particular species of events has always, in all cases, been in conjunction with another, we have no longer any scruple to predict the one from the appearance of the other, nor to employ this reasoning, which alone can assure us of any question of fact or existence. Let us then call one object cause; the other, effect. We suppose that there is some connection between them, some power in the one by which the one infallibly produces the other, and operates with the greatest certainty and the strongest necessity

But there is nothing different in a number of cases from what there is in any singular case to which it is supposed to be exactly similar; except that, after a repetition of similar cases, the mind is led by habit, on the occasion of the occurrence of an event, to expect its usual companion, and to believe that it will exist. ..

If there is a relationship between objects that we care to know perfectly, it is that of cause and effect. On it are founded all our reasonings on questions of fact or existence. Only by means of it do we obtain some certainty about objects far removed from the present testimony of our memory and our feelings. ...

We may, therefore, in accordance with this experience, form another definition of cause, and call it an object followed by another, the appearance of which always leads to the thought of the latter. ....

Every idea is a copy of some preceding impression or feeling; and where we can find no impression, we may be sure there is no idea. In all singular cases of the operation of bodies or minds there is nothing which produces an impression, nor which, consequently, can suggest the idea of necessary power or connection. But when many uniform cases present themselves, and the same object is always seen to be followed by the same event, we begin to have the notion of cause and connection. 

VII. On freedom and necessity

Our idea, therefore, of necessity and causation arises entirely from the uniformity which may be observed in the operations of nature, in which similar objects are constantly conjunct with each other, and the mind is determined by habit to infer one from the appearance of the other...Beyond the constant conjunction of similar objects, and the consequent inference from one to the other, we have no notion of necessity or connection.  

The philosopher, if he is consistent, must apply the same reasoning to the actions and volitions of intelligent agents

VIII. On academic or skeptical philosophy

We always suppose an external universe, which does not depend on our perception, but which would exist even if we and every sentient creature were absent or annihilated. 

This same table that we see white, and that we notice solid, we believe that it exists independently of our perceiving it, and that it is something external to our mind that perceives it. Our presence does not confer it being. Our absence does not annihilate it. It preserves its uniform and complete existence, independently of the situation of the intelligent beings who perceive or contemplate it. 

But this universal and primary opinion of all men is soon destroyed by the slightest philosophy, which teaches us that nothing can be present to the mind but an image or perception, and that the senses are only the channels through which these images are transmitted, without being capable of producing any immediate interaction between mind and object.....

By what argument can it be proved that the perceptions of the mind must be caused by external objects completely different from them though similar to them?

It is a question of fact whether sense perceptions are produced by external objects similar to them. How should this question be resolved? By experience, surely, like all other questions of a similar nature. But here experience is and must be completely mute. The mind never has anything present before it except perceptions and cannot possibly attain to any experience of its connection with objects.

The most profound and philosophical skeptics always succeed when they pretend to introduce a universal doubt... Your reason can never find any convincing argument from experience to prove that perceptions are connected with whatever external objects. 

These principles [of Pyrrhon's skepticism] may flourish and triumph in the schools, where it is indeed difficult, if not impossible, to refute them. But as soon as they leave the shadows, and in the presence of the real objects acted upon by our passions and feelings, they stand in opposition to the most powerful principles of nature and vanish like smoke.

The skeptic, therefore, would do better to remain within his own sphere, and to expose those philosophical objections which arise from deeper investigations Here he seems in a wide field to triumph; so long as he justly insists that all our evidence on any question of fact which lies beyond the testimony of the senses or of memory, is derived entirely from the relation of cause and effect; that we have no other idea of this relation than that of two objects which have been frequently conjunct; that we have no argument whatever to convince us that these objects which have been, in our experience, frequently conjunct, will be equally, in other cases, conjunct in the same manner; and that nothing leads us to this inference but habit and or a certain instinct of our nature..... 

There is certainly a more mitigated skepticism or academic philosophy, which may be both useful and enduring, and may, in part, be the result of this pyrrhonism, or excessive skepticism, when the indiscriminate doubts of the latter are corrected by common sense and reflection....

Another species of mitigated scepticism which may be advantageous to mankind, and which may be the natural result of Pyrrhonian doubts and scruples, is the limitation of our investigations to those matters for which the narrow capacity of the human understanding is best adapted....A correct judgment follows a contrary method, and, avoiding all lofty and distant investigation, confines itself to common life, and to such matters as fall under everyday practice and experience; leaving the most sublime topics for the embellishment of poets and orators, or for the arts of priests and politicians... 26 It seems to me that the only objects of abstract science or demonstration are quantity and number, and that all attempts to extend this more perfect species of knowledge beyond these limits are sophistry and illusion. 

All the rest of man's researches concern questions of fact and existence; and these are evidently not susceptible of demonstration. All that is, may not be.

The existence, therefore, of any being can only be proved by arguments from its cause or its effect; and these arguments are completely founded on experience, [and not] by reasoning a priori....

The sciences he deals with in general fact are politics, natural philosophy, physics, chemistry, etc., in which the qualities, causes and effects of a whole class of objects are investigated. 

Morality and criticism are not so much objects of understanding as of taste and feeling.

When we go through libraries, persuaded of these principles, what havoc will we not wreak! If we take in our hands a volume of theology or scholastic metaphysics, for example, let us ask ourselves: does it contain any abstract reasoning about quantity and number? No. Does it contain any experimental reasoning about matters of fact and experience? No. Throw it then into the flames, for it can contain nothing but sophistry and deception.            

      c) Criticism

David Hume is known, and rightly so, as the philosopher who launched the lethal torpedo on the philosophical carrier of causality, although this had already received the less convincing attack of Malebranche. It is true that his skepticism about causality, and about the existence of an external world whose existence we know of by the impressions that it “causes” in us, does not put him at the radical level of the extreme skepticism of Pyrrhon, which seems to him unassailable in the academy but contradictory when he leaves it, but as a beneficial and moderate skepticism which, while aware of the lack of rational foundation - intuitive and demonstrative - of causality and existence, maintains them as customary beliefs with the pragmatic motivation of conducting one's own life. It is almost the same as radical skepticism, with the only difference that it includes the pragmatic attitude as part of its program, and in fact the intellectual heritage of David Hume understood his philosophy in its most radical sense, that is, as the abolition of causality and of the existence of a world external to us. 

This is very serious, and it is a mortal blow to the philosophical tradition that had reached him. On the few occasions in which the Bible speaks of philosophy, that is, of that which men can know by their natural lights, without the need for revealed data, it makes explicit reference to causality: men come to know God - and must therefore render glory to him - through their works, as we read in Romans 1:20.

David Hume's philosophy will have great influence on later philosophy. Immanuel Kant will say that reading Hume awakened him from his dogmatic sleep. In fact, Kant's philosophy is an effort to save, as an apriorism of knowledge, both causality and the other categories necessary to do philosophy and to do science, after their loss in his shipwreck in Hume's philosophy. It may well be said that there was a Kant because there was a Hume before him. But Kant's offspring will soon notice the contradiction in his philosophy that external reality causes impressions in our sensibility, while affirming that causality has no extramental reality: the solution to this insoluble problem will be Hegelian idealism, which will dispense with reality and therefore with the problem; or else Schopenhauer's philosophy in which the world will be held to be pure mental representation, so that in causing its impressions on our faculty of knowing, that causation will not be between reality and representation but between representation and representation, so that the fact that it itself is a pure representation of our understanding will no longer repugn the fact that it is itself a pure representation of our understanding.

Now, if the world is representation, what is obliged next is to ask what is represented, and what is then answered will occupy the exact place of being, will replace to the self. The answer is suggested by Kant himself, who really recovers external reality, as we conceive it, is in Practical Reason, that is, in the domain of the will: what is represented is the will.  

Being replaced by will in Schopenhauer's philosophy, being dissolved into idea in Hegel's philosophy, these are the points of arrival. Marx will follow Hegel, for it is the same to say “everything is idea”, or “everything is spirit”, as it is to say “everything is matter”, as he points out in the Misery of Philosophy, the decisive thing is that there is no longer any distinction between matter and spirit. And the aforementioned Schopenhauer will be followed by his ardent reader in his youth, Friedrich Nietzsche, to whom all ideas and representations will end up being surplus to requirements in order to be left with only the will, the will to live, what is truly real. but he will take seriously that there is only will, and understood as the will to live, after all, the law by which nature is governed. And this already begins to ring a bell when we remember the political history of the 20th century. 

Hume has been the sulfuric acid in philosophy, and so Hume cannot go unanswered.

In view, then, of the point of arrival we have made, it will be of vital importance, then, that we examine the reasons why Hume rejects causality: “I have found that such an object has always been accompanied by such an effect, and I foresee that other objects, in appearance similar, will be accompanied by similar effects. I will grant, if you please, that one proposition may properly be inferred from the other. Indeed, I know that it is always inferred. But if you insist that the inference is made by a chain of reasonings, I will ask you to reproduce this reasoning (...) When a child has experienced the sensation of pain, by touching the flame of a candle he will be careful not to put his hand near any candle; and he will expect a similar effect from a cause which is similar in its appearance and in its sensible qualities. If you assert, therefore, that the child's understanding is led to this conclusion by a process of argumentation or reasoning, I may with justice require you to reproduce this argument.” 

Let us accept this requirement that Hume makes us by way of a challenge, and let us remember beforehand, as a warm-up, the two hypotheses that Poincaré puts forward as necessary to do science: 

1) When faced with a fact, we always look for the simplest explanation. For example, whenever Kepler observed the position of Mars, he found it on an ellipse, and thus concluded that Mars has that ellipse as its trajectory, in spite of the fact that there are many other curves in space that are not ellipses and that pass through the same observed positions. It seemed natural to him to choose the ellipse, among all those curves (it has degree two) because it is the simplest curve among all those that pass through them. 

2) Nature always responds in the same way to the same circumstances, and therefore, in those same circumstances, it will respond in the future in the way it has done so far.  

I personally believe that this second hypothesis reduces to the first one, since the simplest explanation for the fact that the same result has been obtained so far in an experiment is that it is not a matter of chance upon chance, but that this result had to be necessarily obtained (even if we did not know then, in fact, the reason for such a necessity). Consequently, it must also be so in the future. 

Having reduced Poincaré's discourse to the hypothesis of simplicity, that our reason always seeks the simplest explanation, let us say hypothesis of simplicity or unity, let us say that such hypothesis is not something strange or supervenient to thought, but the very essence of our thinking: to know something, to understand a fact, is to find the unity that is given in it. We say, on the other hand, that we do not understand something when it appears before us as a mosaic of data without any relation between them (this was well understood by the ancients: it was the revelation of the priestess Diotima to Socrates, as he himself narrates in his speech in The Banquet: the sage always seeks simplicity and unity in the inquiries of his thought, and the artist seeks unity, harmony between the parts, in his search for beauty. The revelation of Diotima consisted in the fact that the supreme simplicity and the supreme beauty are one and the same being, and that to adhere to this unique Beauty and unique Truth, is the true and complete way to reach the immortality that we humans have. This was also understood by Kant when he put in the search for simplicity and unity the very essence of human reasoning, and put in fact in the world as unity the pure idea of our reason, one of those three that stimulate it in its speculative discourse. Therefore, this hypothesis of simplicity does not mean any renunciation of knowledge, but is the very essence and presupposition of the use of our reason.

After this preheating, let us now attend to David Hume's injunction: I have found that to such causes - let us leave for the moment whether they really are - the same effects have always followed, hitherto, the same effects. The simpler explanation The most incredible explanation, because it is so complicated, is that it has always happened this way (that iron has always expanded with heat and always with exactly the same coefficient of expansion) by an endless number of coincidences accumulated one on top of the other, always the same result without any reason for it, something that nobody is willing to believe. The conclusion we then draw is that the same thing must therefore also happen in the future.  

And this is also the reasoning of the child: whenever he has approached a flame he has been burned, and although he does not know how to express it, he has understood that it has not been by chance accumulated over chance once, but because it has to be this way -even if he does not know the reason- and therefore he will not approach the fire again. The child has unconsciously searched for the simplest explanation, which we express by saying that he has reasoned, because the search for the simplest explanation, the search for unity, the very essence of reasoning, to the point that without this assumption there is no activity of reason: everything would be admitted as unconnected facts, with nothing to connect.

And we come to Hume's assertion that the reason for that concomitance of facts which experimental science calls cause C and effect E will never be found, as He called heat the cause C of the dilatation of iron, and this dilatation he called the effect E of heat. And he called them thus, cause and effect, before having explained, two centuries later, why C is the cause of E, that is, why E must necessarily follow C. He considered them as such, before having such a demonstration, by virtue of that implicit reasoning that we have just expressed explicitly in response to Hume's challenge, reasoning that gives the simplest explanation of so many coincidences in the past, always dilating iron, always with exactly the same coefficient of dilatation. 

Hume says: “Heat and light are side effects of fire”... ”Who will claim to be able to give the ultimate reason why milk or bread is suitable food for man, and not for a lion or a tiger?” ... “Our senses inform us of the color, weight, and consistency of bread; but neither the senses nor the reason we can never to inform of those qualities which make it suitable for the nourishment and sustenance of a human body” ... ”The bread, which I previously ate, nourished me; that is, a body of such and such sensible qualities was, at that time, endowed with such and such secret powers. But does it follow that another bread, at another time, must likewise nourish me, and that similar sensible qualities must always be accompanied by similar secret powers? The consequence does not seem at all necessary..... But if you insist that the inference is drawn by a hip of reasonings, I will ask you to reproduce this reasoning.”

Well, here is the answer, here is the reasoning: heat and light are effects of fire, which is an oxidation reaction in which heat is produced because, after the reaction, electrons occupy lower energy levels and therefore release energy in the form of radiation. As for light and its color, this is due to the fact that there are electrons vibrating between two energy levels, which is explained because it passes to a higher energy level when absorbing a photon, and then emits a photon of the same frequency passing to a lower energy level: the energy difference in the two levels between which it vibrates coincides exactly with the energy (hν) of the photons that it absorbs and emits. This is therefore a frequency of reflected light. With all the reflected frequencies we obtain the color of the object, in this case the yellowish color of the fire, then the red of the burning log, and finally the absence of color of the black body that remains at the end. This is the current explanation, which according to Hume would never exist. 

As for bread, let us say that it has long starch molecules, which the saliva splits into sucrose - only twelve carbons - and these are then split into two glucoses - only six carbons - until they are split into carbon dioxide - only one carbon - and water, the latter reaction releasing a lot of energy which is stored by passing ADP (adenosine diphosphate) molecules to ATP (adenosine triphosphate) molecules which go to the muscles. When this energy is needed to carry out a movement, the ATP molecules return to ADP molecules with the release of the energy they had stored in the form of a chemical bond, energy which is used to move the muscles, thus becoming kinetic energy. 

All this has been known since the 1960s reaction to reaction, a cycle very similar, by the way - although inverted - to the Krebs cycle of chlorophyll synthesis, since in this one these organic substances are synthesized from water and carbon dioxide, absorbing heat, which reaches the plants from the sun. So much the reactions of inorganic chemistry mentioned when speaking of fire, as the reactions of organic chemistry mentioned when speaking of bread are necessarily derived from chemical principles that in turn are necessarily derived from the number of electrons that there are in the last layer of the atoms that form the molecules involved, which in turn is determined by the number of possible electrons in each shell, which is easily obtained from the principles of quantum mechanics and the mathematical theory of representations of the SU(2) symmetry group (the SO(3) group of rotations, but given the spin, are representations of SU(2), double coating of SO(3)).

"Reason can never inform about those qualities We have already seen that he has done so, and very prolix information indeed, ending in the irreducible representations of the SU(2) symmetry group, and so in every single case, without exception, cited and uncited, of which Hume has said that no reason could ever be found why the so-called effect necessarily follows from what we call cause. The present development of science has been a resounding disavowal of the reason for Hume's assertion that there is no reasoning linking effect with cause, and that it is only our habituation to the temporal contiguity of the two facts.

It may be objected to me that the reasonings provided by science -I have sketched some of them- are in turn based on postulates of science (how not, for if reasonings are required, they cannot be ad infinitum), and that these are in turn universal laws, or rather universalized by the belief that the results in future experiences will be the same as in past experiences, so that what Hume himself says could be applied: “we only delay the line of our ignorance”. I answer again, as a justification of this universalization of experimental and therefore particular statements, with the principle of simplicity: the simplest explanation of the fact that nature has so far responded, under the same conditions, with the same result, is that it had to necessarily come out, under those conditions, with that result, and consequently that same result will come out in the future, and that is what the universal law expresses.

This hypothesis of simplicity, at the basis of the use of our reason, is what makes it rational that many particular judgments -only particular judgments bring experience- come to bring a universal judgment. There is no logical justification, for the particular will never imply in logic the universal, but there is rational justification, and what Hume has asked us to do is to make reasoning explicit. Reason is much more than logic, as Gilbert Chesterton rightly says: the mad are not those who have lost logic, for it is the only thing they retain, but those who have lost reason. 

We have made explicit a reasoning based on quantum mechanics, and, more importantly, a reasoning that would have been already valid at the very time when Hume wrote: to find the simplest explanation. The simplest explanation for the fact that it has always dilated and always with the same coefficient is that this must necessarily be the case. The search for the simplest explanation, the search for unity, is the very essence of our knowledge, for without the presupposition of unity or simplicity in nature, our faculty of knowing has nothing to do: to think is to find unity in what initially seemed various, and the presupposition of unity or presumption of rationality is the stimulus that moves us to think. Without this presupposition of unity and universal harmony, everything is a mosaic of data before our senses and our understanding, with nothing to relate, no fact to explain, chance upon chance in our experiences, with no need for any justification. 

Let us at least say that, once the train of science has been set in motion, everyone can get on - regardless of their philosophy - but what is important is the thinking of those who set the train of science in motion, who were by no means skeptical readers of David Hume - his thinking would have been paralyzing - but bold thinkers like Kepler, fortunately a century earlier, who spoke of the presumption of harmony as a stimulus for the search for laws in the planets, until he found them. This is what he says in the introduction to his work, and this is reflected in its title Harmonices Mundi. And so did modern Kepler, Albert Einstein or Werner Heisenberg, and so many other creators of new human knowledge. 

We have seen that Hume also deals with causality - of course, to say that there is none - in the acts for which we feel responsible: if one stone strikes another, we do not therefore say that the movement of the second is free, then for the same reason, since it is the same thing, the act of my body which follows the order of my will will will not be free but necessary. I believe that if this is taken seriously there is no reason to imprison anyone, for no one is responsible - he is not the author, he is not the cause - of his own acts, and in particular he is not the author of his criminal acts (the only reason that would justify locking up the criminal would be to prevent society from that individual, but this would be the justification of the means - the imprisonment of an innocent person - by reason of its end). 

However, it is not difficult to answer Hume that, indeed, from the decision of my will follows necessarily, as an effect, the movement of the finger that pulls the trigger, but the locus of my freedom is before, for it consists in my being able to decide that or the contrary. Therefore I will be responsible for the death that I may cause by making the decision to pull the trigger. In short: For Hume there is no moral responsibility because it could not be otherwise, once causality is denied, but this statement, which undermines the basis of Ethics, is an error in philosophy.

What is really the reason why Hume has dispensed with causality? To my mind, it is because previously George Berkeley's philosophy had abolished substance, something of which there is no longer any trace in a philosophy limited to mere impressions. Indeed, it is clear that it is not a glow and a heat that carbonizes a paper but something which has these qualities of brightness and heat, among other qualities not all of which are directly perceptible, such as the chemical properties derived from the number of electrons in the last orbital of its atoms, which determines its chemical valences. It is this something which carbonizes the paper in a chemical reaction of combustion, and does so by those chemical properties, among which it is possible to find a necessary connection with the carbonization of the paper. It is therefore clear that if we are not allowed to something that is bright and hot but only from the same impressions of brightness and heat, we have run out of discursive space for causality, for no one will find, indeed, any connection of necessity between the impressions of brightness and heat and the phenomenon of the carbonization of the paper. 

Thus the deep reason for the loss of causality is the elimination of substance. Hume cannot admit something of which we have no clear and distinct idea, or, even more drastically, clear and distinct impression, and it is obvious that of substances we have none, for of substances we perceive only their qualities. This had already led Locke to speak of the uselessness of substances in philosophy. George Berkeley and, following Berkeley, David Hume will take the announced step (chronicle of a death foretold), by dispensing in fact with substances. 

But why this demand for clear and distinct ideas in philosophy, we may ask. We refer then to the analysis already carried out of Locke's philosophy: the clarity of the newborn science of nature was available in that century, and it was a question of emulating it in philosophy. This intention to emulate science, characteristic of all modern philosophy, is clear from the beginning, and it is clear now in Hume, in his demand to stick to impressions, i.e. to mere experiment.

And on the subject of causality, I would like to make a comment, preferably addressed to the reader with a scientific background. It is often heard that the randomness of observations in quantum mechanics is a violation of the principle of causality, and that in this sense the present quantum mechanics would agree with Hume. This is not understanding the collapse of the wave function, or not understanding causality. To explain this in an accessible way let us concentrate, for example, on the observable “position”: we cannot say that a particle is in one place or another, but we only have the (density of) probability that it appears in one place or another (probability cloud) when we make an experiment to determine its position. We do the experiment and it then appears in a place where the probability was non-zero. There is no physical explanation that it appeared precisely in this place and not in another place where it was also non-zero. There is neither in the present physics nor in any subsequent physical theory that refines the knowledge of nature that we now have, since it is an intrinsic randomness. This does not mean that the fact that it has appeared somewhere (that is, that it has collapsed its wave function to a subspace proper to the position operator) has had no cause: the cause has been the interaction of my laboratory with that particle leading to determine its position. What happens is that causality is not necessarily deterministic causality, and in fact in this case it is not. (To explain the first statement, let us say that the causality I exercise as the author of my moral actions is not deterministic, but free causality, and therefore I am responsible for them; but the causality of fire in charring a sample of paper is certainly deterministic). 

In fact, this discovery of modern science offers an exciting topic for philosophical reflection - not that it is in itself a scientific topic - from this interesting plot of reality presented by quantum mechanics: As it is known, Einstein was opposed to it because he understood that everything that happens must have a explanation. In particular, there must be an as yet unknown physics - which he called “physics of hidden variables” - that we would one day discover, and which would explain why the particle appears in one place instead of in another, both of which are probable. Well, Bell's inequalities, the violation of which could settle the question, may have been the subject of experience some time after the disappearance of the brilliant physicist - but opposed to the standard interpretation of quantum mechanics - and the experimental violation of these inequalities has disproved Einstein: randomness is intrinsic, and no such subsequent physics of “hidden variables” can be expected, so that Albert Einstein's statement is refuted, understood as a statement in physics, i.e. as a requirement of a "hidden variable physics". physical explanation of everything that happens. But taken as a philosophical affirmation - the “principle of sufficient reason” according to which everything that happens has explanation, whether we know it or not, is something with which it is impossible to disagree, for the contrary is repugnant to the mind itself and to rationality itself, and so we are forced to conclude that there is more to the explanation of material reality than mere physical explanation. 

There is thus more reality than merely physical reality, and that reality can interact with physical reality to the point of explaining physical facts, such as the movement of my muscles. The explanation of the physical reality of my finger having pulled the trigger instead of sparing a life is explanation by reality in me rather than mere physical reality. We can call it a suggestion of immateriality (=nonphysical reality) of the human spirit, or at least call it a doorway that indeterminacy physics leaves open to affirmation philosophical that our actions are not determined but are determined by our free will, and therefore we are responsible for them. This is also a suggestion that science leaves the door open to the possibility that God can be provident without changing the laws of physics, but rather acting through them.

We have concentrated so far on the critique of causality, but we have seen that its denial leads to crude skepticism even regarding the existence of an external reality, a reality that is independent of our own perceptions (the concept of substance had already been dissolved, as we have said, by George Berkeley, and therefore, since we cannot speak of beings, ontological causality or causality in being, cannot even be glimpsed. In fact, his discourse on causality refers only to physical causes). It is true that Hume opts for a skepticism that accepts external reality, independent of my own being, as pure customary belief, since the radical skeptical stance, unassailable in school, seems to him paralyzing and inadvisable for life, as we have already said. But apart from the fact that any distinction between academia and life - if philosophy is to deal with reality and life - must be vigorously denied, this realism of pragmatic reasons seems hardly sustainable as a philosophical stance, and in fact Hume's inheritance is radical skepticism, even if he was sincere in not claiming it. But one cannot help but wonder, for what reason is it unassailable in school? As Aristotle says, if the skeptic says nothing, he does not bother; and if he says something, he is self-refuting (though perhaps the skeptic can say something irrefutable: he can say “I am going to do this”, without further explanation, without further justification. That is why skepticism is frightening, and its inheritance is frightening: it is at the basis of nihilism, which in turn is at the basis of totalitarianism).

But the problem with Hume is that he said something, for in fact he wrote the work we have commented on. That is why Aristotle's criticism can be applied to him, as to every skeptic: Hume ends the book by saying that there is no valid knowledge other than that which deals with relations between ideas, referring to arithmetic and geometry, or that knowledge which deals with matters of fact, by which he means experimental sciences, i.e. knowledge of phenomena by their causes, but understood as an accumulation of experiences of contiguity of phenomena. And everything beyond that, including morals, are feelings, but not true knowledge. If we are persuaded of these principles, as Hume says in his concluding words, we must throw into the fire all treatises that do not deal with geometry or arithmetic or any experimental science, for they can contain nothing but sophistry and deception. Let us take the book Research on Human Understanding Does it deal with numbers? No, no formula to be seen. Does it deal with facts of experience? No, neither, not a single graph that collects data. Well then, let us throw it into the fire, for it can contain nothing but sophistry and deception.

But we have considered that it contains the thoughts of one person, and as such it has deserved our respect and our comments.