THE COMPOSITION OF CAUSES.
An effect is almost always the result of the concurrence of several
causes. When all have their full effect, precisely as if they had
operated _successively_, the joint effect (and it is not inconsistent to
give the name of _joint effect_ even to the mutual obliteration of the
separate ones) may be _deduced_ from the laws which govern the causes
when acting separately. Sciences in which, as in mechanics, this
principle, viz. the _composition of causes_, prevails, are deductive and
demonstrative. Phenomena, in effect, do generally follow this principle.
But in some classes, e.g. chemical, vital, and mental phenomena, the
laws of the elements when called on to work together, cease and give
place to others, so that the joint effect is not the sum of the separate
effects. Yet even here the more general principle is exemplified. For
the new _heteropathic_ laws, besides that they never supersede _all_ the
old laws (thus, The weight of a chemical compound is equal to the sum of
the weight of the elements), have been often found, especially in the
case of vital and mental phenomena, to enter _unaltered_ into
composition with one another, so that complex facts may thus be
_deducible_ from comparatively simple laws. It is even possible that, as
has been already partly effected by Dalton's law of definite
proportions, and the law of isomorphism, chemistry itself, which is now
the least deductive of sciences, may be made deductive, through the laws
of the combinations being ascertained to be, though not compounded of
the laws of the separate agencies, yet derived from them according to a
fixed principle.
The proposition, that effects are proportional to their causes, is
sometimes laid down as an independent axiom of causation: it is really
only a particular case of the composition of causes; and it fails at the
same point as the latter principle, viz. when an addition does not
become compounded with the original cause, but the two together generate
a new phenomenon.