Page:Popular Science Monthly Volume 29.djvu/558

542 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 rendered evident through simple induction by means of mere enumeration. 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 middle of the seventeenth century that the fundamental principle of the law of universal causation began to take root among the natural scientists, as the impression gained ground that in nature like conditions necessitate the taking place of like phenomena. Since then, this fundamental 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 apparatus, 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 conditions 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 phenomenon 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 mundi, 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 conditions 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