Suppose a person present with me, who advances propositions, to which I do not assent, _that Cæsar died in his bed, that silver is more fusible than lead, or mercury heavier than gold_; 'tis evident that, notwithstanding my incredulity, I clearly understand his meaning, and form all the same ideas which he forms. My imagination is endowed with the same powers as his; nor is it possible for him to conceive any idea, which I cannot conceive; or conjoin any, which I cannot conjoin. I therefore ask, wherein consists the difference betwixt believing and disbelieving any proposition? The answer is easy with regard to propositions, that are proved by intuition or demonstration. In that case, the person who assents not only conceives the ideas according to the proposition, but is necessarily determined to conceive them in that particular manner, either immediately, or by the interposition of other ideas. Whatever is absurd is unintelligible; nor is it possible for the imagination to conceive any thing contrary to a demonstration. But as, in reasonings from causation, and concerning matters of fact, this absolute necessity cannot take place, and the imagination is free to conceive both sides of the question, I still ask, _wherein consists the difference betwixt incredulity and belief_? since in both cases the conception of the idea is equally possible and requisite. 'Twill not be a satisfactory answer to say, that & person, who does not assent to a proposition you advance; after having conceived the object in the same manner with you, immediately conceives it in a different manner, and has different ideas of it. This answer is unsatisfactory; not because it contains any falsehood, but because it discovers not all the truth. 'Tis confessed that, in all cases wherein we dissent from any person, we conceive both sides of the question; but as we can believe only one, it evidently follows, that the belief must make some difference betwixt that conception to which we assent, and that from which we dissent. We may mingle, and unite, and separate, and confound, and vary our ideas in a hundred different ways; but 'till there appears some principle, which fixes one of these different situations, we have in reality no opinion: and this principle, as it plainly makes no addition to our precedent ideas, can only change the _manner_ of our conceiving them. All the perceptions of the mind are of two kinds, viz. impressions and ideas, which differ from each other only in their different degrees of force and vivacity.