we conceive it.

Suppose a person present with me, who advances propositions, to which I
do not assent, that Caesar dyed in his bed, that silver is more
fusible, than lead, or mercury heavier than gold; it is 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; nor 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 deference
betwixt incredulity and belief? since in both cases the conception of
the idea is equally possible and requisite.

It will not be a satisfactory answer to say, that a 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 falshood, but because it discovers not all
the truth. It is contest, 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 until 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