the effect or
phenomenon depends upon causes of two kinds: the properties of the
object acting, and those of the object acted upon. Everything which can
possibly happen in which man and external things, are jointly concerned,
results from the joint operation of a law or laws of matter, and a law
or laws of the human mind. Thus the production of corn by human labour
is the result of a law of mind, and many laws of matter. The laws of
matter are those properties of the soil and of vegetable life which
cause the seed to germinate in the ground, and those properties of the
human body which render food necessary to its support. The law of mind
is, that man desires to possess subsistence, and consequently wills the
necessary means of procuring it.

Laws of mind and laws of matter are so dissimilar in their nature, that
it would be contrary to all principles of rational arrangement to mix
them up as part of the same study. In all scientific methods, therefore,
they are placed apart. Any compound effect or phenomenon which depends
both on the properties of matter and on those of mind, may thus become
the subject of two completely distinct sciences, or branches of science;
one, treating of the phenomenon in so far as it depends upon the laws of
matter only; the other treating of it in so far as it depends upon the
laws of mind.

The physical sciences are those which treat of the laws of matter, and
of all complex phenomena in so far as dependent upon the laws of matter.
The mental or moral sciences are those which treat of the laws of mind,
and of all complex phenomena in so far as dependent upon the laws of
mind.

Most of the moral sciences presuppose physical science; but few of the
physical sciences presuppose moral science. The reason is obvious. There
are many phenomena (an earthquake, for example, or the motions of the
planets) which depend upon the laws of matter exclusively; and have
nothing whatever to do with the laws of mind. Many, therefore, of the
physical sciences may be treated of without any reference to mind, and
as if the mind existed as a recipient of knowledge only, not as a cause
producing effects. But there are no phenomena which depend exclusively
upon the laws of mind; even the phenomena of the mind itself being
partially dependent upon the physiological laws of the body. All the
mental sciences, therefore, not excepting the pure science of mind, must
take account of a great variety of physical truths;