Hume, who has taken up Berkeley’s arguments on this subject, and affirms that the doctrine of abstract ideas applies the flattest of all contradictions, that it is possible for the same thing to be and not to be, has enlarged a good deal on this last topic of the manner in which words may be supposed to excite general ideas. His words are these: ‘Where we have found a resemblance between any two objects that often occur to us, we apply the same name to all of them, whatever differences we may observe in the degrees of their quantity and quality, and whatever differences may appear among them. After we have acquired a custom of this kind, the hearing of that name revives the idea of one of these objects, and makes the imagination conceive it with its particular circumstances and proportions. But as the same word is supposed to have been frequently applied to other individuals that are different in many respects from the idea which is immediately present to the mind, the word not being able to revive the idea of all these individuals, only _touches_ the soul, if I may be allowed so to speak, and revives that custom, which we have acquired by surveying them. They are not in reality present to the mind, but only in power, nor do we draw them out distinctly in the imagination, but keep ourselves in readiness to survey any of them, as we may be prompted by a present design or necessity. The word raises up an individual idea, along with a certain custom; and that custom produces any other individual one, for which we may have occasion.’ Treatise of Human Nature, p. 43, 4. The author afterwards adds, with his usual candour, that this account does not perfectly satisfy him, but he relies principally on the logical demonstration of the impossibilities of abstract ideas just before given.