Summary: A Treatise of Human Nature (page 8)

It is not a question of Hume ceasing to use the notion of cause. It is a matter of proposing a redefinition of it, minimal and psychologising:

We define a cause to be, An object precedent and contiguous to another, and so united with it in the imagination, that the idea of the one determines the mind to form the idea of the other, and the impression of the one to form a more lively idea of the other ¹.

This is all we will hear - and in fact already do hear - when we use the term cause.

Like humans, beasts certainly never perceive any real connexion among objects. ’Tis therefore by experience they infer one from another ².

For example, from the tone of voice the dog infers his master's anger, and foresees his own punishment. From a certain sensation affecting his smell, he judges his game not to be far distant from him ³.

In reality, man acts no differently. The same processes guide him in his inferences. The traditional distinction between human reason and animal instinct thus disappears:

Reason is nothing but a wondeful and unintelligible instinct in our souls, which carries us along a certain train of ideas, and endows them with particular qualities ⁴.

Hume thus criticises the ontological dignity traditionally accorded to reason. Man is not external to and superior to nature. He is an integral part of nature, because his mind is guided by that natural process which is habit:

Nature may certainly produce whatever can arise from habit: Nay, habit is nothing but one of the principles of nature, and derives all its force from that origin ⁵.

Part 4: Of the Sceptical and Other Philosophical Systems

In this final part of Book I, Hume attacks certain philosophical doctrines, current in his day. It is a question for him, at the same time, of presenting his own doctrine in its singularity: an original, modern scepticism, quite different from the ancient scepticism that can be found among the Greeks.

Thus, from the very first lines, we encounter the heart of this sceptical critique: All knowledge degenerates into probability ⁶.

It is based, quite simply in the first instance, on the inconstancy of our mental powers ⁷.

Thus, while in all demonstrative sciences the rules are certain and infallible, [...] when we apply them, our fallible and uncertain faculties are very apt to depart from them, and fall into error ⁸.

However, in the same breath, he attacks the demonstrative sciences themselves, those that seem to reach a certain, necessary and universal truth: the mathematics.

There is no algebraist nor mathematician so expert in his science, as to place entire confidence in any truth immediately upon his discovery of it, or regard it as any thing, but a mere probability ⁹.

Then we find the same pattern as in his critique of causality: the habit of repeated observation will cause us to slide insensitively from probability to certainty. A psychological process leads us to an erroneous belief, that of having reached a universal rule:

Every time he runs over his proofs, his confidence encreases; but still more by the approbation of his friends; and is rais'd to its utmost perfection by the universal assent and applauses of the learned world. Now ’tis evident, that this gradual encrease of assurance is nothing but the addition of new probabilities, and is deriv'd from the constant union of causes and effects, according to past experience and observation ¹⁰.

The movement leads us towards an idea that is more and more probable, but from there to knowledge there is a leap that nothing can allow: Knowledge and probability are of such contrary and disagreeing natures, that they cannot well run insensibly into each other, and that because they will not divide, but must be either entirely present, or entirely absent ¹¹.

However, Hume defends himself from being one of those sceptics, who hold that all is uncertain, and that our judgment is not in any thing possest of any measures of truth and falsehood ¹².

What distinguishes him from them is that he rejects the suspension of judgement advocated by ancient scepticism.

In fact, he considers for his part that judgement is for man a natural faculty, and hence it is as illusory to ask him to suspend this faculty as to cease breathing: Nature, by an absolute and uncontroulable necessity, has determin'd us to judge, as well as to breathe and feel ¹³.

Summing up: "total" scepticism, a kind of "fantastic sect" ¹⁴ directs us towards suspension of judgement.

Hume takes the opposite direction: he shows how the vividness of an idea is strengthened, as a result of repeated impressions, and that thus we give it our assent, so that a belief becomes fortified in the mind.

The sceptic must admit the existence of bodies, that is, of an external world, even if he cannot prove it rationally. This idea is too vivid in us to be denied. This is how Hume opposes the sceptic, using the same method, and it is worth noting the originality of this approach. According to Hume, the total, ancient sceptic cannot deny what he claims to deny. There is no way for him to realise the programme of scepticism.

What we can ask all the same is what made this idea so vivid, i.e. what causes induce us to believe in the existence of body ¹⁵.

For in reality, after these seemingly hostile oratorical precautions to scepticism, Hume will concede to it the essential point, namely that nothing authorises us to believe that there is an external world, independent of our perceptions.

This is why Hume can be classed among the sceptics, and even seen as one of the first modern sceptics, despite his denials.

¹ 1.3.14, p.116
² 1.3.16, p.119
³ Ibid.
⁴ P.120
⁵ Ibid.
⁶ 1.4.1, p.121
⁷ Ibid.
⁸ Ibid.
⁹ Ibid.
¹⁰ Ibid.
¹¹ Ibid.
¹² P.123
¹³ Ibid.
¹⁴ Ibid.
¹⁵ 1.4.2, p.125