LAW OF CAUSATION. 405 



however, of any phenomenon, a special enumeration 

 of which would generally be very prolix, may be all 

 summed up under one head, namely, the absence of pre- 

 venting or counteracting causes. The convenience of 

 this mode of expression is grounded mainly upon the 

 fact, that the effects of any cause in counteracting ano- 

 ther cause may in most cases be, with strict scientific 

 exactness, regarded as a mere extension of its own 

 proper and separate effects. If gravity retards the 

 upward motion of a projectile, and deflects it into a 

 parabolic trajectory, it produces, in so doing, the very 

 same kind of effect, and even (as mathematicians 

 know) the same quantity of effect, as it does in its 

 ordinary operation of causing the fall of bodies when 

 simply deprived of their support. If an alkaline 

 solution mixed with an acid destroys its sourness, 

 and prevents it from reddening vegetable blues, it is 

 because the specific effect of the alkali is to combine 

 with the acid, and form a compound with totally 

 different qualities. This property, which causes of all 

 descriptions possess, of preventing the effects of other 

 causes by virtue (for the most part) of the same laws, 

 according to which they produce their own*, enables 



* There are a few exceptions; for there are some properties of 

 objects which seem to be purely preventive; as the property of 

 opaque bodies, by which they intercept the passage of light. This, 

 so far as we are able to understand it, appears an instance not of 

 one cause counteracting another by the same law whereby it pro- 

 duces its own effects, but of an agency which manifests itself in no 

 other way than in defeating the effects of another agency. If we 

 knew upon what other relations to light, or upon what peculiarities 

 of structure, opacity depends, we might find that this is only an 

 apparent, not a real, exception to the general proposition in the 

 text. In any case it needs not affect the practical application. 

 The formula which includes all the negative conditions of an effect 

 in the single one of the absence of counteracting causes, is not 



