ILLUSTRATIONS OF THE LOGIC OF SCIENCE. 3 



theory of gases. Though unable to say what the movements of any 

 particular molecule of a gas would be on a certain hypothesis regard- 

 ing the constitution of this class of bodies, Clausius and Maxwell 

 were yet able, by the application of the doctrine of probabilities, to 

 predict that in the long run such and such a proportion of the mole- 

 cules would, under given circumstances, acquire such and such veloci- 

 ties ; that there would take place, every second, such and such a num- 

 ber of collisions, etc. ; and from these propositions were able to deduce 

 certain properties of gases, especially in regard to tbeir heat-relations. 

 In like manner, Darwin, while unable to say what the operation of 

 variation and natural selection in any individual case will be, demon- 

 strates that in the long run they will adapt animals to their circum- 

 stances. Whether or not existing animal forms are due to such ac- 

 tion, or what position the theory ought to take, forms the subject of 

 a discussion in which questions of fact and questions of logic are curi- 

 ously interlaced. 



II. 



The object of reasoning is to find out, from the consideration of 

 what we already know, something else which we do not know. Con- 

 sequently, reasoning is good if it be such as to give a true conclusion 

 from true premises, and not otherwise. Thus, the question of its 

 validity is purely one of fact and not of thinking. A being the 

 premises and B the conclusion, the question is, whether these facts 

 are really so related that if A is B is. If so, the inference is valid; 

 if not, not. It is not in the least the question whether, when the 

 premises are accepted by the mind, we feel an impulse to accept the 

 conclusion also. It is true that we do generally reason correctly by 

 nature. But that is an accident ; the true conclusion would remain 

 true if we had no impulse to accept it ; and the false one would re- 

 main false, though we could not resist the tendency to believe in it. 



We are, doubtless, in the main logical animals, but we are not 

 perfectly so. Most of us, for example, are naturally more sanguine 

 and hopeful than logic would justify. We seem to be so constituted 

 that in the absence of any facts to go upon we are happy and self- 

 satisfied; so that the effect of experience is continually to contract 

 our hopes and aspirations. Yet a lifetime of the application of this 

 corrective does not usually eradicate our sanguine disposition. Where 

 hope is unchecked by any experience, it is likely that our optimism is 

 extravagant. Logicality in regard to practical matters is the most 

 useful quality an animal can possess, and might, therefore, result from 

 the action of natural selection ; but outside of these it is probably of 

 more advantage to the animal to have his mind filled with pleasing 

 and encouraging visions, independently of their truth; and thus, 

 upon unpractical subjects, natural selection might occasion a falla- 

 cious tendency of thought. 



