ows himself to be perfectly secure from falling, by his experience of
the solidity of the iron, which supports him; and though the ideas of
fall and descent, and harm and death, be derived solely from custom and
experience. The same custom goes beyond the instances, from which it is
derived, and to which it perfectly corresponds; and influences his
ideas of such objects as are in some respect resembling, but fall not
precisely under the same rule. The circumstances of depth and descent
strike so strongly upon him, that their influence can-not be destroyed
by the contrary circumstances of support and solidity, which ought to
give him a perfect security. His imagination runs away with its object,
and excites a passion proportioned to it. That passion returns back
upon the imagination and inlivens the idea; which lively idea has a
new influence on the passion, and in its turn augments its force and
violence; and both his fancy and affections, thus mutually supporting
each other, cause the whole to have a very great influence upon him.

But why need we seek for other instances, while the present subject
of philosophical probabilities offers us so obvious an one, in the
opposition betwixt the judgment and imagination arising from these
effects of custom? According to my system, all reasonings are nothing
but the effects of custom; and custom has no influence, but by
inlivening the imagination, and giving us a strong conception of
any object. It may, therefore, be concluded, that our judgment and
imagination can never be contrary, and that custom cannot operate on
the latter faculty after such a manner, as to render it opposite to the
former. This difficulty we can remove after no other manner, than by
supposing the influence of general rules. We shall afterwards take
[Sect. 15.] notice of some general rules, by which we ought to regulate
our judgment concerning causes and effects; and these rules are formed
on the nature of our understanding, and on our experience of its
operations in the judgments we form concerning objects. By them we learn
to distinguish the accidental circumstances from the efficacious causes;
and when we find that an effect can be produced without the concurrence
of any particular circumstance, we conclude that that circumstance makes
not a part of the efficacious cause, however frequently conjoined with
it. But as this frequent conjunction necessity makes it have some effect
on the imagination, in spite