.

The deductive method is the main source of our knowledge of complex
phenomena, and the sole source of all the theories through which vast
and complicated facts have been embraced under a few simple laws. It
consists of processes of Induction, Ratiocination, and Verification.
First, by one of the four inductive methods, the simple laws (whence may
be _deduced_ the complex) of each separate cause which shares in
producing the effect, must be first ascertained. This is difficult, when
the causes or rather tendencies cannot be observed singly. Such is the
case in physiology, since the different agencies which make up an
organized body cannot be separated without destroying the phenomenon;
consequently there our sole resource is to produce experimentally, or
find (as in the case of diseases), pathological instances in which only
one organ at a time is affected. Secondly, when the laws of the causes
have been found, we calculate the effect of any given combination of
them by ratiocination, which may have (though not necessarily) among its
premisses the theorems of the sciences of number and geometry. Lastly,
as it might happen that some of the many concurring agencies have been
unknown or overlooked, the conclusions of ratiocination must be
_verified_; that is, it must be explained why they do not, or shown that
they do, accord with _observed_ cases of at least equal complexity, and
(which is the most effectual test) that the empirical laws and
uniformities, if any, arrived at by direct observation, can be deduced
from and so accounted for by them, as, e.g. Kepler's laws of the
celestial motions by Newton's theory.

CHAPTERS XII. AND XIII.

THE EXPLANATION AND EXAMPLES OF THE EXPLANATION OF LAWS OF NATURE.

The aim, in the deductive method, is either to discover the law of the
effect, or to account for it by _explaining_ it, that is, by pointing
out some more general phenomenon (though often less familiar to us) of
which this is a case and a partial exemplification, or some laws of
causation which produce it by their joint or successive action. This
explanation may be made, either--1. By resolving the laws of the complex
effect into its elements, which consist as well of the separate laws of
the causes which share in producing it, as also of their collocation,
i.e. the fact that these separate laws have been so combined; or--2. By
resolving the law which connects two links, not proximate, in a chain of
causat