, that it is most at home. The
conception can indeed be carried into the Biological and Social
Sciences, even in its quantitative form, by making the proper
allowances. For the limbs of animals are levers, and act upon mechanical
principles; and digestion and the aeration of the blood by breathing are
partly chemical processes. There is a quantitative relation between the
food a man eats and the amount of work he can do. The numbers of any
species of plant or animal depend upon the food supply. The value of a
country's imports is equal to the value of its exports and of the
services it renders to foreigners. But, generally, the less experiment
and exact calculation are practicable in any branch of inquiry, the less
rigorously can the conception of causation be applied there, the more
will its application depend upon the qualitative marks, and the more
need there will be to use it judiciously. In every inquiry the greatest
possible precision must be aimed at; but it is unreasonable to expect in
any case more precise proof than the subject admits of in the existing
state of culture.

Wherever mental action is involved, there is a special difficulty in
applying the physical notion of causation. For if a Cause be conceived
of as matter in motion, a thought, or feeling, or volition can be
neither cause nor effect. And since mental action is involved in all
social affairs, and in the life of all men and animals, it may seem
impossible to interpret social or vital changes according to laws of
causation. Still, animals and men are moving bodies; and it is
recognised that their thoughts and feelings are so connected with their
movements and with the movements of other things acting upon them, that
we can judge of one case by another; although the connection is by no
means well understood, and the best words (such as all can agree to use)
have not yet been found to express even what we know about it. Hence, a
regular connection being granted, I have not hesitated, to use
biological and social events and the laws of them, to illustrate
causation and induction; because, though less exact than chemical or
mechanical examples, they are to most people more familiar and
interesting.

In practical affairs, it is felt that everything depends upon causation;
how to play the fiddle, or sail a yacht, or get one's living, or defeat
the enemy. The price of pig-iron six months hence, the prospects of the
harvest, the issue in a Coroner's Cou