COMPOSITION OF CAUSES. 



243 



This condition is realised in the ex- 

 tensive and important class of pheno- 

 mena commonly called mechanical, 

 namely, the phenomena of the com- 

 munication of motion (or of pressure, 

 which is tendency to motion) from 

 one body to another. In this impor- 

 tant class of cases of causation, one 

 cause never, properly speaking, defeats 

 or frustrates another ; both have their 

 full effect. If a body is propelled in 

 two directions by two forces, one tend- 

 ing to drive it to the north and the 

 other to the east, it is caused to move 

 in a given time exactly as far in both 

 directions as the two forces would 

 separately have carried 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 dynamics, the principle of 

 the Composition of Forces : and, in 

 imitation of that well-chosen expres- 

 sion, I shall give the name of the 

 Composition of Causes to the prin- 

 ciple which is exemplified in all cases 

 in which the joint efifect of several 

 causes is identical with the sum of 

 their separate effects. 



This principle, however, by no 

 means prevails in all departments of 

 the field of nature. The chemical 

 combination of two substances pro- 

 duces, as is well known, a third sub- 

 stance with properties different from 

 those of either of the two substances 

 separately, or of both of them taken 

 together. Not a trace of the proper- 

 ties of hydrogen or of oxygen is ob- 

 servable in those of their compound, 

 water. The taste of sugar of lead is 

 not the sum of the tastes of its com- 

 ponent elements, acetic acid and lead 

 or its oxide ; nor is the colour of blue 

 vitriol a mixture of the colours of sul- 

 phuric acid and copper. This ex- 

 plains why mechanics is a deductive 

 or demonstrative science, and chemis- 

 try not. In the one, we can com- 

 pute the effects of combinations of 

 causes, whether real or h3rpothetical, 

 from the laws which we know to 

 govern those causes when acting sepa- 



rately, because they continue to ob- 

 serve the same laws when in combina- 

 tion which they observed when sepa- 

 rate : whatever would have happened 

 in consequence of each cause taken by 

 itself, happens when they are together, 

 and we have only to cast up the 

 results. Not so 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 alto- 

 gether when they are conjoined ; and 

 we are not, at least in the present 

 state of our knowledge, able to fore- 

 see what result will follow from any 

 new combination, until we have tried 

 the specific experiment. 



If this be true of chemical com- 

 binations, it is still more true of those 

 far more complex combinations of 

 elements which constitute organised 

 bodies, and in which those extra- 

 ordinary new uniformities arise which 

 are called the laws of life. All orga- 

 nised bodies are composed of parts 

 similar to those composing inorganic 

 nature, and which have even them- 

 selves existed in an inorganic state ; 

 but the phenomena of life 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 know- 

 ledge of the properties of the several 

 ingredients 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 we ever predict that it could 

 taste, unless gelatine or fibrin could 

 themselves taste ; for no elementary 

 fact can be in the conclusion which 

 was not in the premises. 



