Summary: A Treatise of Human Nature (page 8)

The point is not that Hume ceases to use the notion of cause. Rather, he proposes a redefinition of it—minimal and psychological:

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 mean—and in fact already do mean—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 behaves no differently. The same processes guide his inferences. The traditional distinction between human reason and animal instinct thus dissolves:

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

Hume thus challenges the ontological dignity traditionally accorded to reason. Man is not external to nature, nor superior to it. 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 takes aim at certain philosophical doctrines current in his day. His purpose is at the same time to set out his own doctrine in its distinctiveness: an original, modern scepticism, quite different from the ancient scepticism found among the Greeks.

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

This rests, 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.

In the same breath, however, he attacks the demonstrative sciences themselves—those that seem to attain certain, necessary, and universal truth: 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.

Here we find the same pattern as in his critique of causality: the habit of repeated observation will lead us imperceptibly from probability to certainty. A psychological process drives us towards an erroneous belief—that of having arrived at 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 carries us towards an ever more probable idea, but from there to knowledge there is a leap that nothing can justify: 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.

Hume is nonetheless at pains to distance himself from 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 sets him apart is his rejection of the suspension of judgement advocated by ancient scepticism.

He holds that judgement is a natural faculty in man, and that asking him to suspend it is therefore as illusory as asking him to stop breathing: Nature, by an absolute and uncontroulable necessity, has determin'd us to judge, as well as to breathe and feel ¹³.

In short: "total" scepticism—a kind of "fantastic sect" ¹⁴—points towards the suspension of judgement.

Hume takes the opposite path: he shows how the vividness of an idea is strengthened through repeated impressions, and how we thereby give it our assent, so that a belief becomes consolidated in the mind.

The sceptic must acknowledge 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 turns the sceptic's own method against him, and the originality of this move is worth noting. The thoroughgoing ancient sceptic, on Hume's account, cannot deny what he claims to deny: there is no way for him to carry out the sceptical programme.

What we may still ask is what made this idea so vivid—that is, what causes induce us to believe in the existence of body ¹⁵.

For in reality, after these seemingly anti-sceptical rhetorical precautions, Hume will concede to scepticism its essential point: that nothing authorises us to believe in an external world independent of our perceptions.

This is why Hume may be counted among the sceptics, and even regarded as one of the first modern sceptics, despite his disclaimers.

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