286 THE POPULAR SCIENCE MONTHLY. 



ages of certain other diseases will in this case be the very conditions 

 to promote the ravages of cholera. A parallel case would be that of 

 carefully removing the coals of fire from a building every night, as a 

 safeguard to the structure ; but let a sudden gale spring up, and the 

 embers thus removed would be scattered far and wide. 



ILLUSTRATIONS OF THE LOGIC OF SCIENCE. 



By C. S. PE1KCE, 



ASSISTANT IN THE UNITED STATES COAST SVRVET. 



SECOND PAPER. HOW TO MAKE OUR IDEAS CLEAR. 



I. 



WHOEVER has looked into a modern treatise on logic of the 

 common sort, will doubtless remember the two distinctions 

 between clear and obscure conceptions, and between distinct and con- 

 fused conceptions. They have lain in the books now for nigh two 

 centuries, unimproved and unmodified, and are generally reckoned by 

 logicians as among the gems of their doctrine. 



A clear idea is defined as one which is so apprehended that it will 

 be recognized wherever it is met with, and so that no other will be 

 mistaken for it. If it fails of this clearness, it is said to be obscure. 



This is rather a neat bit of philosophical terminology ; yet, since it is 

 clearness that they were denning, I wish the logicians had made their 

 definition a little more plain. Never to fail to recognize an idea, and 

 under no circumstances to mistake another for it, let it come in how rec- 

 ondite a form it may, would indeed imply such prodigious force and 

 clearness of intellect as is seldom met with in this world. On the other 

 hand, merely to have such an acquaintance with the idea as to have 

 become familiar with it, and to have lost all hesitancy in recognizing it 

 in ordinary cases, hardly seems to deserve the name of clearness of 

 apprehension, since after all it only amounts to a subjective feeling of 

 mastery which may be entirely mistaken. I take it, however, that 

 when the logicians speak of " clearness," they mean nothing more 

 than such a familiarity with an idea, since they regard the quality as 

 but a small merit, which needs to be supplemented by another, which 

 they call distinctness. 



A distinct idea is defined as one which contains nothing which 

 is not clear. This is technical language ; by the co?itents of an idea 

 logicians understand whatever is contained in its definition. So that 

 an idea is distinctly apprehended, according to them, when we can 

 give a precise definition of it, in abstract terms. Here the profes- 

 sional logicians leave the subject ; and I would not have troubled the 



