Summary: A Treatise of Human Nature (page 5)

Part 3: Of Knowledge and Probability

Curiously, geometry is not for Hume an exact science, owing to the fact that the objects it examines extend in space: Geometry, or the art, by which we fix the proportions of figures; tho' it much excels both in universality and exactness, the loose judgments of the senses and imagination; yet never attains a perfect precision and exactness ¹.

He reserves this title for only two sciences: There remain, therefore, algebra and arithmetic as the only sciences, in which we can carry on a chain of reasoning to any degree of intricacy, and yet preserve a perfect exactness and certainty ².

It is in the next section that we encounter one of the most important passages in A Treatise of Human Nature, in which Hume develops an idea that has caused scandal down the centuries: his critique of the notion of causality.

One might go so far as to argue that it constitutes the heart of the work, and that it is thanks to it that the work has secured its place in the history of philosophy.

It is probably one of the starting points of the Critique of Pure Reason, as Kant strives to restore the causal relation to its full legitimacy against the Humean critique.

Let us turn at once to this celebrated doctrine that challenges the notion of causality.

Between fire and smoke, there is a causal relationship: fire is the cause of smoke. What leads us to posit such a relationship between two things?

Hume identifies two necessary conditions: contiguity and succession.

- Contiguity: we cannot conceive of a causal relationship between two objects that are too far apart in space and time.

- Succession: the cause precedes the effect.

Yet we cannot stop there:

An object may be contiguous and prior to another, without being consider'd as its cause. There is a NECESSARY CONNEXION to be taken into consideration; and that relation is of much greater importance, than any of the other two above-mention'd ³.

Let us apply the Humean method we are now familiar with, tracing an idea back to the impression from which it came:

Here again I turn the object on all sides, in order to discover the nature of this necessary connexion, and find the impression, or impressions, from which its idea may be deriv'd ⁴.

This necessary connection comes into sharpest focus in two questions, which it is therefore worth examining:

- For what reason we pronounce it necessary, that every thing whose existence has a beginning, shou'd also have a cause? ⁵ — a concise formulation of what is called the principle of causality.

- Why we conclude, that such particular causes must necessarily have such particular effects; and what is the nature of that inference we draw from the one to the other, and of the belief we repose in it? ⁶.

For Hume, certainty can rest on four kinds of relations: contrariety, resemblance, degree of quality, and proportion of number.

However, the first principle — Whatever begins to exist, must have a cause of existence ⁷ — does not rest on any of these, so that proposition therefore is not intuitively certain ⁸.

Nor can any reasoning come to ground this principle. For example, suppose we are told: if nothing is the cause of the existence of something, then nothingness is the cause of another being, which is impossible since nothingness has no properties. The reply will be: we are not claiming that nothingness is the cause of this thing; we are arguing that it has no cause.

If we can rely neither on intuition nor on reasoning, perhaps we can rely on experience. But which experience?

To infer an effect from a cause, and thus establish the existence of that cause, we must rely on an impression. Only an impression can ground this existence and legitimise this inference.

We must therefore get to grips with what matters here: First, The original impression. Secondly, The transition to the idea of the connected cause or effect. Thirdly, The nature and qualities of that idea ⁹.

The ultimate cause of impressions that come from the senses is, for Hume, perfectly inexplicable by human reason ¹⁰. Is it the object itself? The creative power of the mind? God? In the end, it matters little for the question at hand.

We must first examine what leads us to believe in an idea, to give our assent to it — as is the case here with the principle of causality.

Note that we do not believe in ideas that come from the imagination, nor do we give them our assent, unlike those that come from memory or the senses.

What distinguishes the ideas furnished by these two faculties? A difference of strength, of intensity, of "vividness": The ideas of the imagination [are] fainter and more obscure ¹¹.

We may therefore conclude:

The belief or assent, which always attends the memory and senses, is nothing but the vivacity of those perceptions they present; and that this alone distinguishes them from imagination ¹².

This is what is at work in us when we believe in a causal relationship — when we give our assent to the idea that everything that exists must have a cause.

To sum up, this preparatory examination has uncovered a fundamental principle: assent springs from the vividness of perceptions.

The foundation of the causal relation is not to be found in things themselves: There is no object, which implies the existence of any other if we consider these objects in themselves, and never look beyond the ideas which we form of them ¹³.

Were that the case, fire would simply be smoke itself.

The principle of causality is an inference that takes us beyond the examination of the thing itself. What, then, can it rest on? On experience: 'Tis therefore by EXPERIENCE only, that we can infer the existence of one object from that of another ¹⁴.

But which experience?

¹ 1.3.1, p.51
² Ibid.
³ 1.3.2, p.55
⁴ Ibid.
⁵ Ibid.
⁶ Ibid.
⁷ 1.3.3, p.56
⁸ Ibid.
⁹ 1.3.5, p.59
¹⁰ Ibid.
¹¹ P.60
¹² P.61
¹³ 1.3.6, ibid.
¹⁴ Ibid.