Summary: A Treatise of Human Nature (page 11)

Sensible impressions are given successively in time: an impression of whiteness is followed by another impression of whiteness.

And because the succession is rapid, the mind takes it for identity:

'Tis evident, that as the ideas of the several distinct successive qualities of objects are united together by a very close relation, the mind, in looking along the succession, must be carry'd from one part of it to another by an easy transition, and will no more perceive the change, than if it contemplated the same unchangeable object ¹.

From this easy transition, the imagination infers an identity: Any such succession of related qualities is readily consider'd as one continu'd object, existing without any variation ².

Now let us shift our angle of view, and consider these variations at more distant moments in time—say, a month later. Then the variations, which were insensible when they arose gradually, do now appear of consequence, and seem entirely to destroy the identity ³.

Here we find the same pattern as before: a contradiction arises, a tension that the imagination tries to resolve by forging a reconciling fiction—that of substance.

Let us take the different moments one by one:

a) The contradiction:

By this means there arises a kind of contrariety in our method of thinking, from the different points of view, in which we survey the object, and from the nearness or remoteness of those instants of time, which we compare together.

When we gradually follow an object in its successive changes, the smooth progress of the thought makes us ascribe an identity to the succession; because 'tis by a similar act of the mind we consider an unchangeable object.

When we compare its situation after a considerable change the progress of the thought is broke; and consequently we are presented with the idea of diversity ⁴.

b) The resolution of the contradiction:

In order to reconcile which contradictions the imagination is apt to feign something unknown and invisible, which it supposes to continue the same under all these variations; and this unintelligible something it calls a substance ⁵.

Thus is born the idea of substance—a reconciling idea, and therefore a consoling one, since it holds together both change and permanence, and hence identity.

Substance therefore has no ontological reality: it is a pure product of the imagination, designed to reconcile two opposing tendencies in the mind.

Substance resolves a second contradiction as well: between the compound nature of the parts we perceive (colour, flavour, shape, and so on) and the simplicity of the object. Substance serves as a principle of union or cohesion among these qualities, and as what may give the compound object a title to be call'd one thing, notwithstanding its diversity and composition ⁶.

From the idea of substance derive other equally fictitious notions—substantial form, accidents, occult qualities—which must be definitively abandoned, just like those of sympathies, antipathies and horrors of a vacuum ⁷.

The world is thus emptied of its objects: with Hume, we are left with mere collections of qualities, which nothing can legitimately gather into a single object. Distinct qualities, existing separately, with no subject of inhesion to sustain and support them ⁸:

Every quality being a distinct thing from another, may be conceiv'd to exist appart, and may exist apart, not only from every other quality, but from that unintelligible chimera of a substance ⁹.

By the time Hume wrote these lines, the notion of substance was already coming under criticism from some philosophers—a characteristic feature of modern philosophy, the philosophy of his day. We have already mentioned Berkeley, but Locke too rejects this notion, replacing it with a conceptual pairing he considers far more workable: that of primary and secondary qualities.

For Hume, this is yet another fiction of the imagination, as he sets out to demonstrate in the following section, entitled "Of Modern Philosophy".

According to this doctrine, colours, sounds, tastes, smells, heat and cold are nothing but impressions in the mind, deriv'd from the operation of external objects, and without any resemblance to the qualities of the objects ¹⁰.

This conception rests on the idea that the same quality cannot produce two different impressions—yet what tastes bitter to one person seems sweet to another, and so on.

Primary qualities, by contrast—extension, solidity, figure, motion, and so on—are alone held to be real.

So the generation, encrease, decay, and corruption of animals and vegetables, are nothing but changes of figure and motion ¹¹.

In the end, it all comes down to this: One figure and motion produces another figure and motion; nor does there remain in the material universe any other principle, either active or passive, of which we can form the most distant idea ¹².

For Hume, instead of explaining the operations of external objects by its means, we utterly annihilate all these objects, and reduce ourselves to the opinions of the most extravagant scepticism concerning them ¹³.

Hume shows that motion is inconceivable in itself, independently of the idea of a body in motion. And a body is inconceivable without the ideas of extension or solidity, yet 'tis impossible to conceive extension, but as compos'd of parts, endow'd witho colour or solidity ¹⁴.

In fact after the exclusion of colours, sounds, heat and cold from the rank of external existences, there remains nothing, which can afford us a just and consistent idea of body ¹⁵.

The modern doctrine of primary and secondary qualities is thus rendered unintelligible.

Let us now turn to the nature of the mind. Is it a substance—that is, a medium of inherence for our thoughts, which persists through their flow?

Is this substance material or immaterial? This is the question examined in the next section, aptly entitled "Of the immateriality of the soul".

In reality, we have already disposed of the notion of substance, but Hume takes the time once more to expose its vacuity:

We have no perfect idea of any thing but of a perception. A substance is entirely different from a perception. We have, therefore, no idea of a substance. Inhesion in something is suppos'd to be requisite to support the existence of our perceptions. Nothing appears requisite to support the existence of a perception. We have, therefore, no idea of inhesion ¹⁶.

The question "Is mind an immaterial substance?" is therefore simply unintelligible: We do not so much as understand the meaning of the question ¹⁷.

This traditional philosophical problem is thus resolved—or rather, dismissed outright as ill-formed.

The remainder of the section accordingly offers only a few minor developments, since the problem does not genuinely exist.

We shall therefore pass quickly over these concluding remarks, and move on to Hume's celebrated critique of personal identity: a final section that lands like a thunderclap, and has left its mark on the history of philosophy ever since.

¹ 1.4.3, p.145
² Ibid.
³ Ibid.
⁴ P.145-146
⁵ P.146.
⁶ Ibid.
⁷ P.148
⁸ Ibid.
⁹ P.147
¹⁰ 1.4.4, p.149
¹¹ P.150
¹² Ibid.
¹³ Ibid.
¹⁴ Ibid.
¹⁵ P.151
¹⁶ 1.4.5, p.153-154
¹⁷ P.154