reflection, and which cannot be
prevented by it. Now it is the nature of custom not only to operate with
its full force, when objects are presented, that are exactly the same
with those to which we have been accustomed; but also to operate in an
inferior degree, when we discover such as are similar; and though the
habit loses somewhat of its force by every difference, yet it is seldom
entirely destroyed, where any considerable circumstances remain the
same. A man, who has contracted a custom of eating fruit by the use of
pears or peaches, will satisfy himself with melons, where he cannot find
his favourite fruit; as one, who has become a drunkard by the use of
red wines, will be carried almost with the same violence to white, if
presented to him. From this principle I have accounted for that species
of probability, derived from analogy, where we transfer our experience
in past instances to objects which are resembling, but are not exactly
the same with those concerning which we have had experience. In
proportion as the resemblance decays, the probability diminishes;
but still has some force as long as there remain any traces of the
resemblance.

This observation we may carry farther; and may remark, that though
custom be the foundation of all our judgments, yet sometimes it has an
effect on the imagination in opposition to the judgment, and produces
a contrariety in our sentiments concerning the same object. I explain
myself. In almost all kinds of causes there is a complication of
circumstances, of which some are essential, and others superfluous; some
are absolutely requisite to the production of the effect, and others
are only conjoined by accident. Now we may observe, that when these
superfluous circumstances are numerous, and remarkable, and frequently
conjoined with the essential, they have such an influence on the
imagination, that even in the absence of the latter they carry us on to
t-he conception of the usual effect, and give to that conception a force
and vivacity, which make it superior to the mere fictions of the fancy.
We may correct this propensity by a reflection on the nature of those
circumstances: but it is still certain, that custom takes the start, and
gives a biass to the imagination.

To illustrate this by a familiar instance, let us consider the case of
a man, who, being hung out from a high tower in a cage of iron cannot
forbear trembling, when he surveys the precipice below him, though he
kn