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 into 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 an essential passage in A Treatise of Human Nature, in which Hume develops an idea that will cause 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 found its place in the history of philosophy.

This 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.

Without further ado let us examine this famous doctrine that challenges the notion of causality.

Between fire and smoke, there is a causal relationship: fire is the cause of smoke. Where does it come from that we assume such a relationship between two things?

Hume assumes two necessary conditions: contiguity and succession.

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

- Succession: the cause precedes the effect.

Now 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 follow the Humean method we are familiar with by now, taking 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 is given in a privileged way in two questions, which it is therefore appropriate to examine:

- For what reason we pronounce it necessary, that every thing whose existence has a beginning, shou'd also have a cause? ⁵ - a simple 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 be based on four kinds of relations: contrariety, resemblance, degree of quality and proportion of number.

However, this 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.

Also no reasoning can come to ground this principle. For example, suppose we are told: if nothing is the cause of the existence of something, nothingness is the cause of another being, which is impossible because it has no property. The reply will be: we are not saying that nothingness is the cause of this thing, we are arguing that it has no cause.

If we can rely neither on evidence nor on reasoning, perhaps we can rely on experience. Which one?

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 come to found this existence, and legitimise this inference.

We must therefore grasp as to the subject that matters to us 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 him perfectly inexplicable by human reason ¹⁰. Is it the object? Of the creative power of the mind? Of God? In the end, it matters little for the question under consideration here.

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

Note that we do not believe in the ideas that come out of our imagination, we do not give them our assent, unlike those that come out of our memory or our senses.

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

So we may 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 play in us when we believe in a causal relationship, that we give our assent to this idea that everything that exists must have a cause.

To sum up, this preparatory examination has enabled us to uncover a fundamental principle: assent comes from the vividness of perceptions.

We shall not find the foundation of the relation of causality 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.

If this were the case, fire would be smoke itself.

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

Which one?

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.