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Hume : The attack on Rational Essentialism in the Enquiries Concerning Human Understanding

I
Section IV : Sceptical Doubts Concerning the Operation of the Understanding
The Attack on Rational Essentialism

[The doctrine of rational essentialism is the doctrine that with our reason (hence “rationalism”) we can perceive the essential properties of things (“essentialism”).]
It is in this section that Hume advances his account of the relation between cause and effect. In so doing Hume is expressly opposing Descartes' rationalism. Descartes believed that the whole of physics could be deduced from the operations of pure reason alone — in short, experiment was not required at all to derive the whole body of physics! According to Descartes, every cause carries within it an essence that points to and necessitates the effect that follows from it. This is illustrated by the following headings taken from Descartes Principles.
Principle XXXVI
That God is the First Cause of movement and that He always preserves an equal amount of movement in the universe.

Principle XXXVII
The first law of nature: that each thing as far as it lies, continues always in the same state; and that which is once moved always continues so to move.
This in effect derives a prototype statement of Newton's first law [That an object remains at rest or in uniform motion is a straight line unless acted upon by a resultant force] from the existence of God, and omits any reference to experience (and experiment) in so doing!
Clearly, such a doctrine would contradict the empiricist belief that all ideas are derived from impressions. This is the theory of rational essentialism.
Hume advances in opposition to this theory of rational essentialism his own doctrine that knowledge of the relation of cause and effect is derived from experience alone. He analyses the relation of cause and effect into the constant conjunction of like effects (events) with like causes (events).
To support this claim Hume effectively issues a challenge to anyone to show him an alternative source of the knowledge of cause and effect. He argues that it is only experience that shows us that how one billiard ball may cause a second billiard ball to move.
This proposition, that causes and effects are discoverable not by reason but by experience, will readily be admitted with regard to such objects, as we remember to have once been altogether unknown to us; since we must be conscious of the utter inability, which we then lay under, of foretelling what would arise from them. Present two smooth piece of marble to a man who has no tincture of natural philosophy; he will never discover that they will adhere together in such a manner as to require great force to separate them in a direct line, while they make so small a resistance to a lateral pressure. Such events, as bear little analogy to the common course of nature, are also readily confessed to be known only by experience; nor does any man imagine that the explosion of gunpowder, or the attraction of a loadstone, could ever be discovered by arguments a priori. [By a priori here Hume means reasoning without regard to experience. A priori is contrasted to a posteriori which means, to reason from experience.]
This is effectively written and is a compelling refutation of Descartes' wilder claims. However, the debate does not end there. Clearly, experience is essential in the formation of a scientific law, but is experience alone sufficient to enable the mind to form generalizations? The fact that “the explosion of gunpowder” cannot be discovered by a priori reasoning, does not demonstrate that a priori reasoning has no place whatsoever in science as a whole. Once again it is Kant who argues that science is a product of a mixture of activities — part a priori reasoning and part reasoning from experience (called a posteriori reasoning).
In defence of his view, Hume also puts forward an argument that claims that if the causal relation was known through inference, then a new-born baby should be able to infer it, which is absurd. However, this ignores the fact that children develop consciousness, so that whilst the knowledge of causal relations may be innate it does not follow that new born children are fully conscious of them.

II
The Paradox of Induction

There is, however, a sceptical consequence of this doctrine that has come to be known as the paradox of induction. If all scientific knowledge is based only on experience, then scientific knowledge cannot be certain.
This has been formulated as the problem that from past experience it is not possible to infer what the future will be like. Regularities in the past do not prove that there will be regularities in the future.
As to past Experience, it can be allowed to give direct and certain information of those precise objects only, and that precise period of time, which fell under its cognizance: but why this experience should be extended to future times, and to other objects, which for aught we know, may be only in appearance similar; this is the main question on which I would insist. The bread, which I formerly eat, nourished me; that is, a body of such sensible qualities was, at that time, endued with such secret powers: but does it follow that other bread must also nourish me at another time, and that like sensible qualities must always be attended with like secret powers?
Hume's concerns are different from those of modern authors — for example, Russell. Hume does not appear to be concerned as to whether the paradox of induction undermines science. In his “sceptical solution to these problems” Hume is rejecting the idea that science is objective knowledge. Science is based on custom, or conditioning, and no more. Modern defenders of science have a problem with the paradox of induction because they want to believe that science is cognitive, objective and true, and that, hence, they know that the future will resemble the past, not merely believe it will.
Hume is more concerned to attack Descartes. Because he holds that our knowledge of cause and effect is not based on reason but on instinct, he is not concerned with the sceptical attack the problem of induction could pose for science.
Thus, the problem of induction expresses for Hume the limitations of reason. According to Hume, reason is not a special faculty that can provide knowledge independently of sensation; and so, its powers are limited, perhaps, to the knowledge of its own limitations!
Underlying this conception of our knowledge of the relation of cause and effect is a mechanistic model of the mind. A belief that an effect follows from a cause is mechanically and unconsciously conditioned in us by our past experiences. Hume uses the terms “custom”, “habit” and “instinct” to describe the fundamental nature of the mind.
During this section Hume also touches upon the problem of mathematical knowledge. Hume does not recognize a special problem regarding mathematics and logic as Kant does in his concept of the synthetic a priori. Hume touches upon the problem of applied mathematics, but he maintains that no geometric or mathematical principle can apply to physical reality unless experience shows that it does.
However, Hume's treatment of the philosophy of mathematics is a weakness in his text.

III
Section V: Sceptical Solution of these Doubts
Anticipation of the Theory of Natural Selection

Hume embraces the sceptical consequences of his own theory of knowledge, and then argues that these do not matter. The scepticism derives from the limitations that Hume claims apply to the powers of reason, which are consequences of his empiricism. He claims that in practice these sceptical consequences are irrelevant to life.
Nature will always maintain her rights, and prevail in the end over any abstract reasoning whatsoever.
So no sceptical conclusion will ever have any major impact on our behaviour. It is not possible, merely because one recognizes for example that there is no proof of the uniformity of gravity, to defy one's belief in gravity, for example, by throwing oneself out of the window. Human nature with its determined patterns of behaviour always governs our actions. Scepticism does not affect our actions, and cannot undermine them.
Our actions are not based on reason at all, but upon custom, habit or instinct, and we are mechanically determined.
This principle is Custom or Habit. For wherever the repetition of any particular act or operation produces a propensity to renew the same act or operation, without being impelled by any reasoning or process of the understanding, we always say, that this propensity is the effect of Custom. ... And it is certain we here advance a very intelligible proposition at least, if not a true one, when we assert that, after the constant conjunction of two objects — heat and flame, for instance, weight and solidity — we are determined by custom alone to expect the one from the appearance of the other. ... All inferences from experience, therefore, are effects of custom, not of reasoning.
Hume also claims that our instinctual beliefs are beneficial to us — they enable the species to survive. Reason could never be strong enough to supply us with accurate motives for action, and we are saved from the disastrous effects of relying upon reason alone by our instinct. In this way Hume anticipates the Darwinian theory of natural selection.
Custom, then, is the great guide of human life. It is that principle alone which renders our experience useful to us, and makes us expect, for the future, a similar train of events with those which have appeared in the past. Without the influence of custom, we should be entirely ignorant of every matter of fact beyond what is immediately present to the memory and senses. We should never know how to adjust means to ends, or to employ our natural powers in the production of any effect.
In part II Hume writes
Here, then, is a kind of pre-established harmony between the course of nature and the succession of our ideas; and though the powers and forces, by which the former is governed, be wholly unknown to us; yet our thoughts and concepts have still, we find, gone on in the same train with the other works of nature. Custom is that principle by which this correspondence has been effected; so necessary to the subsistence of our species, and the regulation of our conduct, in every circumstance and occurrence of human life.
In other words, we have evolved the instinctual ability to reason with regard to cause and effect because it is useful to the survival of the species.

IV
Non-cognitivism

To say that our knowledge of cause and effect is a matter of instinctual belief is to say that conscious, cognitive knowledge of causes and effects is not required.
Hume advances a non-cognitivist theory of belief in this section — especially in Part II. This is a consequence of his theory of the relationship of cause and effect being one of constant conjunction. Since that relationship cannot be rationally known as such, the belief in it cannot be cognitive either. In Hume's opinion, belief arises from the way we entertain an idea, with a different degree of strength to when we merely imagine it. A belief is an idea that has a differing degree of vivacity from one that we merely imagine.
I say, then, that belief is nothing but a more vivid, lively, forcible, firm, steady conception of an object, that what the imagination alone is ever able to attain.
Hume has a theory of why we instinctively believe in the relation of cause and effect.
Firstly, to say that one event A causes another B is to say that events that are like A have always been followed in the past by events that are like B. Thus, in this way, the relationship of cause and effect is analysed into the resemblance and continguity.
The constant conjunction of cause and effect reinforces the bond between the two types of event. After many reinforcements, the mind easily passes mechanically from the idea of one event (the cause) to the idea of the other (the effect). Thus, when one considers a cause one is immediately lead, by this species of mechanical association, to the idea of the effect. It is much easier to pass from the cause to the effect than from the cause to some other idea that is merely imagined. Thus the mind becomes conditioned to expect the effect to follow from the cause.
If indeed all belief is mechanically conditioned, why then write a work in philosophy? Nonetheless, Hume does believe that some processes of reasoning are superior to others — they are better adapted to reality. Hume seeks to argue against modes of thinking that he regards as superstitious. Superstitious thinking reinforces unnatural connections between ideas by stressing purely accidental associations based on resemblance, as opposed to genuine causal relations. It may not be possible to demonstrate the validity of induction, but Hume still believes that a science based on induction is infinitely preferable to any other system.

V
Section VI : Of Probability

This section demonstrates Hume's underlying conviction in a deterministic conception of nature. Although we do not know cognitively that nature is uniform, it is in fact uniform — such is his claim. [Hume assumes determinism. That is, that if event A causes event B then, provided the description of the events is sufficiently precise, then event B could not have been caused by anything other than event A.] As a consequence, there can be only one actual outcome to any particular event. So probability is not a real property of events. Hume offers us a psychological theory of probability. Although effects are entirely determined, our knowledge of the causes of effects may in practice be so limited, that a range of outcomes may seem possible. The probability that we will ascribe to an outcome occurring will be in direct proportion to our experiences in the past of like events producing like effects.

VI
Section VII : Of the Idea of Necessary Connection

This section is motivated by Hume's desire to reject Descartes' notion of the interaction of mind and body. Descartes' theory gains strength from the idea that we have an intuitive experience of the actual causal power when a volition is followed by an action. There seems to be a force with which we are directly acquainted that lies in our will and produces (of necessity) the actions that follow it. One feels that the decisions one takes produce the effects that follow them.
Hume offers in this section various counter-arguments against the idea of mind/body interaction. Firstly, there is the claim that the interaction of mind and body is mysterious.
For first; is there any principle in all nature more mysterious than the union of soul with body; by which a supposed spiritual substance acquires such an influence over a material one, that the most refined thought is able to actuate the grossest matter?
Secondly he claims that we do not in fact experience any causal power in the will, but infer that we have causal powers of volition in the same way that we learn about any other relation of cause and effect, that is to say, from experience. We have been able, for instance, in the past, to move a certain object, and we infer that we will always be able to do so.
... that our idea of power is not copied from any sentiment or consciousness of power within ourselves, when we give rise to animal motion, or apply our limbs to their proper use and office. That their motion follows the command of the will is a matter of common experience, like other natural events: but the power or energy by which this is effected, like that in other natural events, is unknown and inconceivable.
This is really the whole point of Hume's attack on rational essentialism. By denying any intuitive (that is, direct) knowledge of the relation of cause and effect Hume has prepared the ground for this attack on the notion of an intuitive awareness of our own power of volition. That we have a power to do things is only known through experience, like any other relation of cause and effect.
Thirdly, Hume points out that the power of the will in Descartes's sense would constitute a power to effectively create something from nothing, and that would overturn the principle of the conservation of matter and energy.
Volition is surely an act of the mind, with which we are sufficiently acquainted. Reflect upon it. Consider it on all sides. Do you find anything in it like this creative power by which it raises from nothing a new idea, and with a kind of Fiat, imitates the omnipotence of its Maker, if I may be allowed so to speak, who called forth into existence all the various scenes of nature.
Thus, he denies the existence of volitions on the grounds that they would be incompatible with determinism.

VI
Other issues

Hume considers the idea advanced by Berkeley that all our ideas are immediately created by God in us. Hume does not offer a refutation of this doctrine, but simply maintains that it is far too speculative to carry any conviction. He claims that it ascribes powers to reason that reason simply does not possess.
Hume also touches upon a powerful sceptical argument at the beginning of section VII. Any inference involves a minute amount of doubt — that is, a nagging doubt based on the feeling that we might have made a mistake in our inference. When inferences are chained together in a long deductive sequence, this doubt increases and undermines our confidence in any conclusion.
Hume also considers what will later become the theory of logical atomism. He draws a distinction between complex ideas and simple ideas, and maintains that complex ideas are no more than the amalgamation of their ultimately simple parts.