throw a piece of dry wood into a fire, my mind is immediately carried to
conceive, that it augments, not extinguishes the flame. This transition
of thought from the cause to the effect proceeds not from reason. It
derives its origin altogether from custom and experience. And as it
first begins from an object, present to the senses, it renders the idea
or conception of flame more strong and lively than any loose, floating
reverie of the imagination. That idea arises immediately. The thought
moves instantly towards it, and conveys to it all that force of
conception, which is derived from the impression present to the senses.
When a sword is levelled at my breast, does not the idea of wound and
pain strike me more strongly, than when a glass of wine is presented to
me, even though by accident this idea should occur after the appearance
of the latter object? But what is there in this whole matter to cause
such a strong conception, except only a present object and a customary
transition to the idea of another object, which we have been accustomed
to conjoin with the former? This is the whole operation of the mind, in
all our conclusions concerning matter of fact and existence; and it is a
satisfaction to find some analogies, by which it may be explained. The
transition from a present object does in all cases give strength and
solidity to the related idea.

Here, then, is a kind of pre-established harmony between the course of
nature and the succession of our ideas; and though the powers and
forces, by which the former is governed, be wholly unknown to us; yet
our thoughts and conceptions have still, we find, gone on in the same
train with the other works of nature. Custom is that principle, by which
this correspondence has been effected; so necessary to the subsistence
of our species, and the regulation of our conduct, in every circumstance
and occurrence of human life. Had not the presence of an object,
instantly excited the idea of those objects, commonly conjoined with it,