OF NATURAL PHILOSOPHY. 151 



of experiment as distinguished from mere passive 

 observation. We make an experiment of the cru- 

 cial kind when we form combinations, and put in 

 action causes from which some particular one shall 

 be deliberately excluded, and some other purposely 

 admitted ; and by the agreement or disagreement 

 of the resulting phenomena with those of the class 

 under examination, we decide our judgment. 



(145.) When we would lay down general rules 

 for guiding and facilitating our search, among a 

 great mass of assembled facts, for their common 

 cause, we must have regard to the characters of 

 that relation which we intend by cause and effect. 

 Now, these are, 



1st, Invariable connection, and, in particular, in- 

 variable antecedence of the cause and conse- 

 quence of the effect, unless prevented by some 

 counteracting cause. But it must be observed, 

 that, in a great number of natural phenomena, 

 the effect is produced gradually, while the cause 

 often goes on increasing in intensity; so that 

 the antecedence of the one and consequence of 

 the other becomes difficult to trace, though it 

 really exists. On the other hand, the effect 

 often follows the cause so instantaneously, that 

 the interval cannot be perceived. In conse- 

 quence of this, it is sometimes difficult to decide, 

 of two phenomena constantly accompanying one 

 another, which is cause or which effect. 

 2d, Invariable negation of the effect with absence 

 of the cause, unless some other cause be ca- 

 pable of producing the same effect. 

 3d, Increase or diminution of the effect, with 



L 1- 



