54 2 THE POPULAR SCIENCE MONTHLY. 



exactly the same conditions, the same natural phenomena will always 

 take place. 



This law is at the present time recognized by all philosophers. 

 There is, however, a dispute as to whether it is true a priori, or 

 whether it can only be proved by experience. John Stuart Mill justly 

 insists that the latter is the case. The correctness of this law is ren- 

 dered evident through simple induction by means of mere enumera- 

 tion. It was only a clear understanding of this law that brought about 

 exact repetition of scientific experiments, and made possible positive 

 predictions of the phenomena that would follow. 



Another class of predictions and generalizations is based on the 

 law of universal causation, or Mill's induction. It was about the mid- 

 dle of the seventeenth century that the fundamental principle of the 

 law of universal causation began to take root among the natural scien- 

 tists, as the impression gained ground that in nature like conditions 

 necessitate the taking place of like phenomena. Since then, this fun- 

 damental principle has gradually come to be general property of all 

 sciences. It has even penetrated into many classes of the people, into 

 the workshop of the mechanic, into the hut of the glass-blower. It 

 is, however, undoubtedly true that with many persons the idea is not 

 clearly brought to consciousness, that thousands of mechanics, miners, 

 etc., act in accordance with it, without being able to express in words 

 what they seem to feel instinctively. If in their work some attempt 

 fails, if the matter turn out differently from what they had expected, 

 nowadays, they will hardly ascribe the failure to some evil spirit who 

 seeks to mock them, but the eyes of the common workman oftentimes 

 will more quickly discover the fault in his appliances and apjDaratus, 

 than the " evil-eye " of the superintendent. 



The essence of Mill's teaching is the empirical deduction of the 

 conception of causes. He has practically evolved this from the law 

 of causation. When an event takes place on a certain combination of 

 conditions, and if this event no longer results when one of these condi- 

 tions is omitted, then this condition is an essential one, a part of the 

 cause. What, then, is the cause of a natural phenomenon ? It is the 

 sum of the essential conditions, in consequence upon which the phe- 

 nomenon invariably follows. Now, it is evident that, if the cause of 

 a natural phenomenon be known, and if this cause occur in any given 

 case, then the effect can be predicted with certainty. This gives us 

 a clearer insight into the theatrum rnundi, so that in many instances 

 we may know on the rising of the curtain what must come. 



If a chemist announces the existence and the properties of a newly 

 discovered substance, for instance, of a new coloring-matter, and we 

 place faith in the accuracy of his work, then we feel convinced that 

 this substance will always be again found whenever the same condi- 

 tions are brought about, although the induction in this case may be 

 based on only a few observations, or may rest perhaps on a single but 



