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. CHAPTER VII. OBSERVATION AND EXPERIMENT.