Summary: A Treatise of Human Nature (page 9)

Hume opens with the great emblematic question of scepticism: whether there really exists an external world corresponding to what we perceive, and with it, the question of the truth of our senses.

Hence the title of this section: "Of scepticism with regard to the senses".

Let us take up the essence of his reasoning, which he sets out rather confusingly over lengthy passages.

The senses present us with external bodies. What we notice, first of all, is that these impressions are discontinuous: I see the tree; when I turn my head, I no longer see it.

Second, these impressions are, as such, internal to the mind.

And yet I am convinced of the continuous and external existence (to the mind) of these objects.

How can this be? This leap, this inference, needs to be justified—and we shall find, as is often the case with Hume, that it cannot be.

Let us therefore examine the two questions more closely:

Why we attribute a CONTINU'D existence to objects, even when they are not present to the senses; and why we suppose them to have an existence DISTINCT from the mind and perception? ¹

Two related questions, since if the objects of our senses continue to exist, even when they are not perceiv'd, their existence is of course independent of and distinct from the perception; and vice versa ².

Let us adopt a genealogical perspective, in search of the origin of this idea of the continuous and external existence of things: does it come from the senses, from reason, or from the imagination?

From the senses: this is impossible, for two reasons.

First, because they convey to us nothing but a single perception, and never give us the least intimation of any thing beyond ³. There must therefore be some inference either of the reason or imagination ⁴.

Furthermore, the senses do not present things to us as external, for to do so they would need to distinguish between the self and the external world. Yet this distinction is obscure even to reason itself:

'Tis certain there is no question in philosophy more abstruse than that concerning identity, and the nature of the uniting principle, which constitutes a person. So far from being able by our senses merely to determine this question, we must have recourse to the most profound metaphysics to give a satisfactory answer to it [...]. 'Tis absurd, therefore, to imagine the senses can ever distinguish betwixt ourselves and external objects ⁵.

It is true, however, that the senses reveal impressions that seem external to our bodies, and so we suppose them also exterior to ourselves ⁶.

For example: the paper, on which I write at present, is beyond my hand. The table is beyond the paper. The walls of the chamber beyond the table ⁷.

Can we not therefore conclude that no other faculty is requir'd, beside the senses, to convince us of the external existence of body ⁸?

No: what I perceive is not my external body but impressions internal to it, so that the ascribing a real and corporeal existence to these impressions, or to their objects, is an act of the mind as difficult to explain, as that which we examine at present ⁹.

Moreover, sounds, tastes, and smells are not spatially extended, yet are nonetheless considered external—which shows that the senses can deceive us.

Conclusion: The opinion of a continu'd and of a distinct existence never arises from the senses ¹⁰.

Nor does this notion come from reason.

To support this, Hume invokes the theory of primary and secondary qualities formulated by Locke and taken up by Descartes.

Reason leads us to distinguish primary qualities (extended bodies, movements, and so on) from secondary qualities (heat, sounds, colours), and tries to convince us that only the former truly exist.

Yet if a philosopher tries to persuade us that the heat of fire does not really exist, he will not be taken seriously.

This shows that we can attribute a distinct continu'd existence to objects without ever consulting REASON, or weighing our opinions by any philosophical principles ¹¹.

Or again: This sentiment, then, as it is entirely unreasonable, must proceed from some other faculty than the understanding ¹².

To find the true origin of this idea, we must turn, in the end, to the imagination.

What leads us to attribute a continuous existence to certain objects is the COHERENCE and CONSTANCY of certain impressions ¹³:

These moutains, and houses, and trees, [...] have always appear'd to me in the same order; and when I lose sight of them by shutting my eyes [...], I soon after find them return upon me without the least alteration ¹⁴.

The question then arises: how do these principles give rise to so extraordinary an opinion? ¹⁵.

In other words: how, from the mere constancy and coherence of a few impressions amid a perpetual flux, do I arrive at something as extraordinary as the existence of a continuous external world independent of the human mind?

Hume begins by pointing to an internal principle of the imagination: The imagination, when set into any train of thinking, is apt to continue, even when its object fails it, and like a galley put in motion by the oars, carries on its course without any new impulse ¹⁶.

Now the same principle makes us easily entertain this opinion of the continu'd existence of body. Objects have a certain coherence even as they appear to our senses; but this coherence is much greater and more uniform, if we suppose the objects to have a continu'd existence; and as the mind is once in the train of observing an uniformity among objects, it naturally continues, till it renders the uniformity as compleat as possible ¹⁷.

In a key passage that deserves to be quoted in full, Hume sets out the heart of the mechanism: a contradiction arises in our mind, and the mind, in order to relieve this tension, forges the idea of a continuous and independent existence:

Nothing is more certain from experience, than that any contradiction either to the sentiments or passions gives a sensible uneasiness, whether it proceeds from without or from within; from the opposition of external objects, or from the combat of internal principles. On the contrary, whatever strikes in with the natural propensities and either externally forwards their satisfaction, or internally concurs with their movements, is sure to give a sensible pleasure.

Now there being here an opposition betwixt the notion of the identity of resembling perceptions, and the interruption of their appearance, the mind must be uneasy in that situation, and will naturally seek relief from the uneasiness. Since the uneasiness arises from the opposition of two contrary principles, it must look for relief by sacrificing the one to the other.

But as the smooth passage of our thought along our resembling perceptions makes us ascribe to them an identity, we can never without reluctance yield up that opinion. We must, therefore, turn to the other side, and suppose that our perceptions are no longer interrupted, but preserve a continu'd as well as an invariable existence, and are by that means entirely the same. ¹⁸.

It is this psychological mechanism—the resolution of a contradiction—that leads us to give our assent to so palpable a contradiction: to suppose a perception to exist without being present to the mind ¹⁹.

¹ 1.4.2, p.125-126
² P.126
³ Ibid.
⁴ Ibid.
⁵ P.127
⁶ Ibid.
⁷ Ibid.
⁸ Ibid.
⁹ Ibid.
¹⁰ P.128
¹¹ P.129
¹² Ibid.
¹³ P.130
¹⁴ Ibid.
¹⁵ Ibid.
¹⁶ P.132
¹⁷ Ibid.
¹⁸ P.136-137
¹⁹ P.137