COMPOSITION OF CAUSES. 211 



the two forces would separately have camcJ it ; and is left precisely 

 where it would have arrived if it had been acted upon first by one of 

 the two forces, and afterwards by the other. This law of nature is 

 called, in mechanical philosophy, the principle of the Composition of 

 Forces : and in imitation of that well-chosen expression, I shall give 

 the name of the Composition of Causes to the principle which is 

 exemplified in all cases in which the joint effect of several causes is 

 identical with the sum of their separate effects. 



This principle; however, by jio means prevails in all departments of 

 the field of natui'e. The chemical combination of two substances 

 produces, as is well known, a third substance with properties entirely 

 different from those of either of the two substances separately, or of 

 both of them taken together. Not a trace of the properties of hydro- 

 gen or of oxygen is observable in those of their compound, water. 

 The taste of sugar of lead is not the sum of the tastes of its component 

 elements, acetic acid and lead or its oxide ; nor is the color of green 

 viti-iol a mixture of the colors of sulphuric acid and copper. This 

 explains why mechanics is a deductive or demonstrative science, and 

 chemisti-y not. In the one, we can compute the effects of all combina- 

 tions of causes, Avhether real or hypothetical, from the laws which we 

 know to govern those causes when apting sej?arately ; because they 

 continue to obsei've the same laws when in combination, which they 

 observed when separate ; whatever would have happened in conse- 

 quence of each cause taken by itself, happens when they are together, 

 and we have only to cast up the results. Not &o in the phenomena 

 which are the peculiar subject of the science of chemistry. There, 

 most of the uniformities to which the causes conformed when separate, 

 cease altogether when they are conjoined ; and we are not, at least in 

 the present state of our knowledge, able to foresee what result will 

 follow from any new combination, until we have tried it by specific 

 experiment. 



If this be true of chemical combinations, it is still more time of those 

 far more complex combinations of elements which constitute organized 

 bodies ; and in which those extraordinary new uniformities arise, which 

 are called the laws of life. All organized bodies are composed of 

 parts, similar to those composing inorganic nature, and which have 

 even themselves existed in an inorganic state ; but the phenomena of 

 iife, which result from the juxtaposition of those parts in a certain 

 manner, bear no analogy to any of the effects which would be produced 

 by the action of the component substances considered as mere physical 

 agents. To whatever degree we might imagine our knowledge of the 

 properties of the several ingi-edients of a living body to be extended 

 and perfected, it is certain that no mere summing up of the separate 

 actions of those elements will ever amount to the action of the living 

 body itself. The tongue, for instance, is, like all other parts of the 

 animal frame, composed of gelatine, fibrin, and other products of the 

 chemistry of digestion, but from no knowledge of the properties of 

 those substances could wc ever predict that it could taste, unless gel- 

 atine or fibrin could themselves taste ; for no elementary fact can be in 

 the conclusion, which was not first in the premisses. 



There are thus two different modes of the conjunct action of causes; 

 from which arise two modes of conflict, or mutual interference, between 

 laws of nature. Suppose, at a given point of time and space, two or 



