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An Enquiry Concerning Human Understanding

35. Suppose a person, though endowed with the strongest faculties of reason and
reflection, to be brought on a sudden into this world; he would, indeed, immediately
observe a continual succession of objects, and one event following another; but he would
not be able to discover anything farther. He would not, at first, by any reasoning, be able
to reach the idea of cause and effect; since the particular powers, by which all natural
operations are performed, never appear to the senses; nor is it reasonable to conclude,
merely because one event, in one instance, precedes another, that therefore the one is the
cause, the other the effect. Their conjunction may be arbitrary and casual. There may be
no reason to infer the existence of one from the appearance of the other. And in a word,
such a person, without more experience, could never employ his conjecture or reasoning
concerning any matter of fact, or be assured of anything beyond what was immediately
present to his memory and senses.
Suppose, again, that he has acquired more experience, and has lived so long in the world
as to have observed familiar objects or events to be constantly conjoined together; what is
the consequence of this experience? He immediately infers the existence of one object
from the appearance of the other. Yet he has not, by all his experience, acquired any idea
or knowledge of the secret power by which the one object produces the other; nor is it, by
any process of reasoning, he is engaged to draw this inference. But still he finds himself
determined to draw it: And though he should be convinced that his understanding has no
part in the operation, he would nevertheless continue in the same course of thinking.
There is some other principle which determines him to form such a conclusion.
36. This principle is Custom or Habit. For wherever the repetition of any particular act or
operation produces a propensity to renew the same act or operation, without being
impelled by any reasoning or process of the understanding, we always say, that this
propensity is the effect of Custom. By employing that word, we pretend not to have given
the ultimate reason of such a propensity. We only point out a principle of human nature,
which is universally acknowledged, and which is well known by its effects. Perhaps we
can push our enquiries no farther, or pretend to give the cause of this cause; but must rest
contented with it as the ultimate principle, which we can assign, of all our conclusions
from experience. It is sufficient satisfaction, that we can go so far, without repining at the
narrowness of our faculties because they will carry us no farther. And it is certain we here
advance a very intelligible proposition at least, if not a true one, when we assert that, after
the constant conjunction of two objects—heat and flame, for instance, weight and
solidity—we are determined by custom alone to expect the one from the appearance of
the other. This hypothesis seems even the only one which explains the difficulty, why we
draw, from a thousand instances, an inference which we are not able to draw from one
instance, that is, in no respect, different from them. Reason is incapable of any such
variation. The conclusions which it draws from considering one circle are the same which
it would form upon surveying all the circles in the universe. But no man, having seen
only one body move after being impelled by another, could infer that every other body
will move after a like impulse. All inferences from experience, therefore, are effects of
custom, not of reasoning7.
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