But philosophers,
observing that, almost in every part of nature, there is contained a
vast variety of springs and principles, which are hid, by reason of
their minuteness or remoteness, find, that it is at least possible the
contrariety of events may not proceed from any contingency in the
cause, but from the secret operation of contrary causes. This
possibility is converted into certainty by farther observation, when
they remark that, upon an exact scrutiny, a contrariety of effects
always betrays a contrariety of causes, and proceeds from their mutual
opposition. A peasant can give no better reason for the stopping of
any clock or watch than to say that it does not commonly go right: But
an artist easily perceives that the same 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.
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