force in the spring or pendulum has always the same influence on the
wheels; but fails of its usual effect, perhaps by reason of a grain of
dust, which puts a stop to the whole movement. From the observation of
several parallel instances, philosophers form a maxim that the connexion
between all causes and effects is equally necessary, and that its
seeming uncertainty in some instances proceeds from the secret
opposition of contrary causes.

Thus, for instance, in the human body, when the usual symptoms of health
or sickness disappoint our expectation; when medicines operate not with
their wonted powers; when irregular events follow from any particular
cause; the philosopher and physician are not surprised at the matter,
nor are ever tempted to deny, in general, the necessity and uniformity
of those principles by which the animal economy is conducted. They know
that a human body is a mighty complicated machine: That many secret
powers lurk in it, which are altogether beyond our comprehension: That
to us it must often appear very uncertain in its operations: And that
therefore the irregular events, which outwardly discover themselves, can
be no proof that the laws of nature are not observed with the greatest
regularity in its internal operations and government.

68. The philosopher, if he be consistent, must apply the same reasoning
to the actions and volitions of intelligent agents. The most irregular
and unexpected resolutions of men may frequently be accounted for by
those who know every particular circumstance of their character and
situation. A person of an obliging disposition gives a peevish answer:
But he has the toothache, or has not dined. A stupid fellow discovers an
uncommon alacrity in his carriage: But he has met with a sudden piece of
good fortune. Or even when an action, as sometimes happens, cannot be
particularly accounted for, either by the person himself or by others;