Viewed in the light of evolution, a law of nature is merely the most generalized expression for a particular occurrence of phenomena. Take, for instance, the law of the conservation of energy: observation, long continued, shows that with whatsoever objects we deal, and-however we may apparently destroy the energy contained in them, yet closer observation, with more accurate instruments, will discern that the energy previously visible has only disappeared to reappear in another form. Finding the same result in every case to which we are able to apply our tests, and discovering no exception to the rule, we abstract the particular objects we have been considering, and, confining our attention to the persistence of energy which each displays, group this class of phenomena into one category and express the likeness by the law that energy endures.
Each deduction from a law is a separate verification of its truth, and as these verifications increase in number the probability of finding an exception decreases. Hence, the law soon assumes a form of necessity as different as possible from its original character. Add to this that many of the laws of nature have only to be expressed to be admitted—laws whose concretes were objects of observation to our earliest ancestors away back in the youth of life upon our globe, and are, to us at least, intuitive—and we see how natural the attribution of necessity to them appears. Besides this, the word law conveys a meaning entirely outside its scientific acceptation. As popularly used it expresses the command of a ruler; and this civil or theological meaning, as applied to the laws of nature, is continually being brought to scientific discussions, much to the detriment of their clearness. Mathematics alone among the sciences has been able to keep clear of these dangerous alliances, and we there still see the word used in its properly scientific application as an order of sequence merely. Although mathematical. law is not coextensive with physical law, it is this meaning which we should endeavor to preserve. The word is an unfortunate one at best, and some philosophers and scientists have advocated its disuse and the substitution of some more accurate term; but it is too deeply rooted in scientific language for that, and we can only enter a protest against its use in scientific discussions in other than a scientific sense. We have only to consider the scientific genesis of the term to obtain a rule for its application. Considered merely as the generalized expression of the result of observation, we clearly perceive that, however long these observations may have continued, they carry with them no necessity except in so far as relates to our own organism. It is just here that the idea of necessity asserts its power. Take the most fundamental law of mechanics, or even (for the supposed necessity in each case arises in the same way) one of the primary axioms of mathematics, and, by an analysis of the genesis of these conceptions, we shall, with the aid of the light that the theory of evolution sheds into those obscure recesses of the mind where consciousness is coming into
