App. 6

Thirdly, We can explain the causes of the firm conception, but not those of any separate impression. And not only so, but the causes of the firm conception exhaust the whole subject, and nothing is left to produce any other effect. An inference concerning a matter of fact is nothing but the idea of an object, that is frequently conjoin'd, or is associated with a present impression. This is the whole of it. Every part is requisite to explain, from analogy, the more steady conception; and nothing remains capable of producing any distinct impression.