Of all the secondary applications of the leading principle of the Treatise, none has perhaps exercised so extensive an influence on philosophy, as this same doctrine of cause and effect. Looking to those separate phenomena, of which in common language we call the one the cause of the other, and the other the effect of that cause, he could see no other connexion between them than that the latter immediately followed the former. He found that the mind, proceeding on the inductive system, when it repeatedly saw two phenomena thus conjoined, expected, when that which had been in use to precede the other made its appearance, that the other would follow; and he found that by repeated experiment this expectation might be so far strengthened, that people were ready to stake their most important temporal interests on the occurrence of the phenomenon called the effect, when that called the cause had taken place. But if there were any thing else but this conjunction, of which a knowledge was demanded--if the unsatisfied investigator sought for some power in the one phenomenon which enabled it to be the fabricator of the other--the sceptical reasoner would answer, that for all he could say to the contrary such a thing might be, but he had no clue to that knowledge--no impression of any such quality passed into his intellect through sensation--his mind had no material committed to it by which the existence or non-existence of any such thing could be argued. The vulgar notion of this theory was, that it destroyed all our notions of regularity and system in the order of nature; that it made no provision for unseen causes, and contemplated only the application of the doctrines of cause and effect to things which were palpably seen following each other. But the inventor of the theory never questioned the regularity of the operations of nature as established by the inductive philosophy; he only endeavoured to show how far and within what limits we could acquire a cognizance of the machinery of that regularity. He denied not that when the spark was applied, the gunpowder would ignite, or that when the ball was dropped, it would proceed to the earth with the accelerated motion of gravitation; but he denied that we could see any other connexion between the cause and effect in either case, than that of uniform sequence. When it was scientifically adopted, the theory was found to be productive of the most important results. The view that when any effect was observed, that phenomenon which was most uniform in its precedence was the one entitled to be termed the cause, was a salutary incentive to close and patient investigation, by laying before the philosopher the simple, numerical question--what was that phenomenon which, by the uniformity of its precedence, was entitled to be termed the cause?[81:1] The test became of the simplest kind; and, if the experimentalist had at a particular time considered some phenomenon