Summary: A Treatise of Human Nature (page 7)

In the next section Hume makes some minor clarifications to the principles just established. He notes, for example, that Contiguity and resemblance have an effect much inferior to causation ¹ on belief in an idea.

He also notes that time reinforces custom, and as a result, our oldest ideas—those instilled in us in childhood—are the ones we hold most firmly.

All those opinions and notions of things, to which we have been accustom'd from our infancy, take such deep root, that 'tis impossible for us, by all the powers of reason and experience, to eradicate them ².

Hence the importance of education: I am perswaded, that upon examination we shall find more than one half of those opinions, that prevail among mankind, to be owing to education, and that the principles, which are thus implicitly embrac'd over-ballance those, which are owing either to abstract reasoning or experience ³.

Surprising as it may seem, we must admit that most of our reasoning with all our actions and passions derives from nothing but custom and habit ⁴.

Hume then turns to the notion of chance, examining merely probable knowledge—that in which evidence [...] is still attended with uncertainty ⁵.

How does one come to believe in an idea that is only probable? The question is, by what means a superior number of equal chances operates upon the mind, and produces belief or assent ⁶.

To shed light on this, Hume describes precisely what happens in the mind when we roll a die. If four sides bear the same number, and two another, then the impulses of the former are, therefore, superior to those of the latter. But as the events are contrary, and 'tis possible both these figures can be turn'd up; the impulses likewise become contrary, and the inferior destroys the superior, as far as its strength goes ⁷.

This is why we do not give our full assent to the idea that we will get this or that number when we roll the dice.

At the end of these considerations on probabilities, here are the two principles worth retaining:

There is nothing in any object, consider'd in itself, which can afford us a reason for drawing a conclusion beyond it.

Even after the observation of the frequent or constant conjunction of objects, we have no reason to draw any inference concerning any object beyond those of which we have had experience ⁸.

In the light of this novel psychological insight, reason loses its primary and normative status: According to my system, all reasonings are nothing but the effects of custom ⁹.

It is in section XIV that Hume mounts a frontal attack on the idea of necessary connection, on which the idea of causality ultimately rests—to say that there is a causal relation between A and B is in effect to suppose that A and B are necessarily connected.

In fact, no genuinely new elements will emerge here. All the pieces of the puzzle are already in place, and in this section Hume brings them together, focusing exclusively on causality.

Hume recapitulates the mechanism: I see that A is followed by B, and after a frequent repetition, I find that upon the appearance of one of the objects, the mind is determin'd by custom to consider its usual attendant [...]. 'Tis this impression [...] which affords me the idea of necessity ¹⁰.

Philosophers have sought to determine what defines a cause as such, with the notions of efficacy, agency, power, force, energy, necessity, connexion and productive quality ¹¹.

We must set aside their circular definitions—or rather, instead of searching for the idea in these definitions, [we] must look for it in the impressions, from which it is originally deriv'd ¹².

As we have seen, reason cannot ground the principle of causality—the idea that a cause is required for any beginning of existence.

Nor is it an innate idea, a notion already refuted, and now almost universally rejected in the learned world ¹³.

Philosophers differ on the question of what makes a cause effective—what enables it to act as such and produce an effect. They have forged the notions of substantial form, accidents and qualities, matter and form, virtue, and so on.

So many obscure, perfectly unintelligible and inexplicable ¹⁴ notions.

Thus we may conclude that 'tis impossible in any one instance to show the principle, in which the force and agency of a cause is plac'd ¹⁵.

If we maintain, as the Cartesians do, that the cause of motion resides not in matter but in God, we run into the same difficulty: no impression is furnished to us of how God communicates motion to extended matter.

All ideas are deriv'd from, and represent impressions. We never have any impression, that contains any power or efficacy. We never therefore have any idea of power ¹⁶.

For Hume, it is the repetition of similar cases that leads us to suppose a causal link, but such repetition can never alone give rise to an original idea ¹⁷, such as that of power or force which defines a cause as such.

To sum up: Similar instances are still the first source of our idea of power or necessity; at the same time that they have no influence by their similarity either on each other, or on any external object ¹⁸.

In the end, it is a process in the mind that leads us to forge the idea of a necessary connection: These instances are in themselves totally distinct from each other, and have no union but in the mind, which observes them, and collects their ideas. Necessity, then, is the effect of this observation, and is nothing but an internal impression of the mind, or a determination to carry our thoughts from one object to another. ¹⁹.

Hence this startling, revolutionary conclusion: the idea of necessary connection—and therefore of causality—is a purely subjective notion:

There is no internal impression, which has any relation to [necessity], but that propensity, which custom produces, to pass from an object to the idea of its usual attendant. This therefore is the essence of necessity. Upon the whole, necessity is something, that exists in the mind, not in objects; nor it is possible for us ever to form the most distant idea of it, consider'd as a quality in bodies ²⁰.

Hume is well aware that this paradox is the most violent ²¹ of all those he presents in this treatise.

With a touch of mischief, he even gives voice to the appalled reactions it is bound to provoke:

What! The efficacy of causes lie in the determination of the mind! As if causes did not operate entirely independent of the mind, and wou'd not continue their operation, even tho' there was no mind existent to contemplate them, or reason concerning them. Thought may well depend on causes for its operation, but not causes on thought. This is to reverse the order of nature and make that secondary, which is really primary ²².

Yet this, however disconcerting, is the conclusion Hume reaches.

¹ 1.3.9, p.76
² p.80
³ P.80-81
⁴ 1.3.10, p.81
⁵ 1.3.11, p.86
⁶ P.88
⁷ P.89
⁸ 1.3.12, p.95
⁹ 1.3.13, p.101
¹⁰ 1.3.14, p.105
¹¹ P.106
¹² Ibid.
¹³ Ibid.
¹⁴ P.107
¹⁵ Ibid.
¹⁶ P.108
¹⁷ P.110
¹⁸ P.111
¹⁹ Ibid.
²⁰ P.112
²¹ Ibid.
²² P.113