Summary: A Treatise of Human Nature (page 6)

It is because we have often seen fire followed by smoke that we think the fire is the cause of the smoke.

In this example, we encounter three elements: succession and contiguity, but also habit. It is habit, in fact, that reinforces the vividness of an idea and consequently our belief in it.

These are precisely the three elements that lead us to postulate a causal relationship between two things, and it is this set that Hume calls the "constant conjunction".

Let us recapitulate the whole process:

We experience fire and find that it is followed by smoke: succession and contiguity.

This experience has been repeated several times, and this habit reinforces, as we have seen, the vividness of our impression, then of our idea "fire is the cause of smoke"...

And thereby our belief in this idea, since assent to an idea depends on its vividness.

From the foregoing it appears that we deduce a necessary connection between two things from their "CONSTANT CONJUNCTION":

Thus we remember to have seen that species of object we call flame, and to have felt that species of sensation we call heat. We likewise call to mind their constant conjunction in all past instances. Without any farther ceremony, we call the one cause and the other effect, and infer the existence of the one from that of the other ¹.

However, if this constant conjunction is at the origin of our idea of causality, it does not constitute a very solid foundation:

This new-discover'd relation of a constant conjunction seems to advance us but very little in our way. For it implies no more than this, that like objects have always been plac'd in like relations of contiguity and succession; and it seems evident, at least at first sight, that by this means we can never discover any new idea ².

In other words: from the constant conjunction, we can only deduce the constant conjunction, and not that of necessary connection, at the heart of the relation of causality. From "A and B have been seen together on several occasions", we cannot deduce "A is the cause of B".

Hume concludes, pitilessly: From the mere repetition of any past impression, even to infinity, there never will arise any new original idea, such as that of a necessary connexion ³.

And yet he enjoins us to remain hopeful... can we not rely on this simple principle, whispered to us by reason: Instances, of which we have had no experience, must resemble those, of which we have had experience ⁴?

In reality this reasoning is only probable, not certain, since we can at least conceive a change in the course of nature ⁵.

Once again, causality, the only [relation] on which we can found a just inference from one object to another ⁶ remains a mere fiction of a mind deceived by the power of habit, which reinforces the vividness of an idea, and thus our belief in it.

Finally, if it is argued that in fire resides the power to create smoke, the problem remains the same: there is nothing to prove to us that this power is always in the object, still less that this power always has the same effect.

What becomes clear to us now is that the principle of causality has no rational basis, but finds its origin in three simply psychological principles that govern the association of ideas: succession, contiguity and habit.

In the next section, Hume turns to the notion of existence. Indeed when we believe in something, we grant it existence.

Hume makes it clear that the idea of existence adds nothing to the idea of an object: The idea of existence is nothing different from the idea of any object, and [...] when after the simple conception of any thing we wou'd conceive it as existent, we in reality make no addition to or alteration on our first idea ⁷.

Where does belief in an idea come from? Not because we add existence to it, since, as we have just seen, the idea of existence adds nothing, but because, as we have seen, the idea is vivid and linked to a present impression:

An opinion, therefore, or belief may be most accurately defin'd, A LIVELY IDEA RELATED TO OR ASSOCIATED WITH A PRESENT IMPRESSION ⁸.

Hume claims to be the first to explain belief in an idea, or assent: This act of the mind has never yet been explain'd by any philosopher ⁹.

Hume describes the mechanism of the association of ideas in more detail. Thus, he asserts to establish [...] as a general maxim in the science of human nature this principle: When any impression becomes present to us, it is not only transports the mind to such ideas as are related to it, but likewise communicates to them a share of its force and vivacity ¹⁰.

He takes the example of Catholic "childishness": the devotees of that strange superstition ¹¹ feel the need to represent the objects of their faith in sensible types and images ¹². These images strengthen their faith.

In Hume, the notion of custom acquires primary importance, more so than in any other doctrine:

Now as we call every thing CUSTOM, which proceeds from a past repetition, without any new reasoning or conclusion, we may establish it as a certain truth, that all the belief, which follows upon any present impression, is deriv'd solely from that origin. When we are accustom'd to see two impressions conjoin'd together, the appearance or idea of the one immediately carries us to the idea of the other ¹³.

This one appears as an irrepressible force: The custom operates before we have time for reflection. The objects seem so inseparable, that we interpose not a moment's delay in passing from the one to the other ¹⁴.

It may even be noted that in some cases the reflection produces the belief without the custom ¹⁵. Indeed, when we have a brand new experience, when we are dealing with the unprecedented, we will nevertheless tend to generalise it without waiting for confirmation.

In fact, we have many millions [experiments] to convince us of this principle; that like objects, plac'd in like circumstances, will always produce like effects; and as this principle has establish'd itself by a sufficient custom, it bestows and evidence and firmness on any opinion, to which it can be apply'd ¹⁶.

¹ 1.3.6, p.61
² P.62
³ Ibid.
⁴ Ibid.
⁵ Ibid.
⁶ P.63.
⁷ 1.3.7, p.65-66
⁸ P.67.
⁹ Ibid, note
¹⁰ 1.3.8, p.69
¹¹ P.70
¹² Ibid.
¹³ P.72
¹⁴ Ibid.
¹⁵ P.73
¹⁶ Ibid.