together, the appearance or idea of the one immediately carries us to the
idea of the other.

Being fully satisfyed on this head, I make a third set of experiments, in
order to know, whether any thing be requisite, beside the customary
transition, towards the production of this phaenomenon of belief. I
therefore change the first impression into an idea; and observe, that
though the customary transition to the correlative idea still remains, yet
there is in reality no belief nor perswasion. A present impression, then,
is absolutely requisite to this whole operation; and when after this I
compare an impression with an idea, and find that their only difference
consists in their different degrees of force and vivacity, I conclude
upon the whole, that belief is a more vivid and intense conception of an
idea, proceeding from its relation to a present impression.

Thus all probable reasoning is nothing but a species of sensation. It is
not solely in poetry and music, we must follow our taste and sentiment,
but likewise in philosophy. When I am convinced of any principle, it is
only an idea, which strikes more strongly upon me. When I give the
preference to one set of arguments above another, I do nothing but decide
from my feeling concerning the superiority of their influence. Objects
have no discoverable connexion together; nor is it from any other
principle but custom operating upon the imagination, that we can draw any
inference from the appearance of one to the existence of another.

It will here be worth our observation, that the past experience, on which
all our judgments concerning cause and effect depend, may operate on our
mind in such an insensible manner as never to be taken notice of, and may
even in some measure be unknown to us. A person, who stops short in his
journey upon meeting a river in his way, foresees the consequences of his