1 66 THE SCIENCE OF LOGIC 



Again, when we are sure that we have taken note of all the 

 circumstances that usually accompany a phenomenon, and set 

 ourselves to discriminate between those that are merely casual or 

 accidental, and those that are causal or essential, we soon advert 

 to the necessity of securing instances that differ from one another 

 in various respects. Instances that vary among themselves are 

 superior in value to instances that are practically similar to one 

 another, because the former enable us to eliminate as unessential 

 to the phenomenon the facts or circumstances which we noticed 

 to be absent in this, that, or the other, of the varied instances in 

 which the phenomenon occurred : the important principle under 

 lying this mode of elimination being that whatever circumstances 

 can be removed or eliminated without interfering with the happening 

 of a phenomenon are not causally connected with that phenomenon. 

 It is here that the superiority of experiment over simple obser 

 vation first becomes manifest. No doubt, in very numerous de 

 partments of research, nature presents us not only with similar 

 instances, but also with varied instances, of the occurrence of a 

 phenomenon. But it is when we can control the agencies of nature, 

 and modify them by experiment, that we can secure the most 

 fruitful variety of instances, the most fruitful combinations of 

 circumstances in which a phenomenon may or may not occur. 

 Experiment thus enables us actively to interrogate and cross- 

 question natural events, to analyse them more effectively, and to 

 disentangle more successfully the connexions which are casual 

 from those that are causal. The raw material furnished by nature, 

 in our sense experience, for scientific analysis and interpretation, 

 is for the most part chaotic and complex. The simple observer 

 has to take this material as he finds it ; the experimenter can 

 control and modify it, and determine for himself the special con 

 ditions under which his observation of it will take place. 



The manner and order in which the experimenter is to handle 

 his materials and to operate upon them, must be left largely to 

 his own scientific knowledge, insight, and genius. His experi 

 ments will be guided by the hypotheses he has formed ; but, like 

 the actual formation of the latter, so, too, the actual procedure in 

 the former cannot be subjected to the guidance of any set of 

 mechanical rules. Logic can only analyse his procedure, and 

 point to some general principles of which that procedure is usually 

 an embodiment and application. Such is the principle under 

 lying the variation of instances in which a phenomenon occurs 



