BOOK I PART III
the imagination; and the resemblance, along
with the constant union, conveys this force and
vivacity to the related idea; which we are there-
fore said to believe, or assent to. If you weaken
either the union or resemblance, you weaken
the principle of transition, and of consequence
that belief, which arises from it. The vivacity of
the first impression cannot be fully conveyed
to the related idea, either where the conjunc-
tion of their objects is not constant, or where
the present impression does not perfectly re-
semble any of those, whose union we are ac-
customed to observe. In those probabilities of
chance and causes above-explained, it is the
constancy of the union, which is diminished;
and in the probability derived from analogy,
it is the resemblance only, which is affected.
Without some degree of resemblance, as well
as union, it is impossible there can be any rea-