EXPLANATION OF LAWS. 525 



only similar in quality, but, when in the same circumstances, 

 identical in quantity, are cases of one and the same law of 

 causation. But the tendency of the moon to the earth, and 

 the tendency of the earth and planets to the sun, were already 

 known to he cases of the same law of causation : and thus the 

 law of all these tendencies, and the law of terrestrial gravity, 

 were recognised as identical, and were subsumed under one 

 general law, that of gravitation. 



In a similar manner, the laws of magnetic phenomena have 

 more recently been subsumed under known laws of electricity. 

 It is thus that the most general laws of nature are usually 

 arrived at : we mount to them by successive steps. For, to 

 arrive by correct induction at laws which hold under such an 

 immense variety of circumstances, laws so general as to be 

 independent of any varieties of space or time which we are 

 able to observe, requires for the most part many distinct sets of 

 experiments or observations, conducted at different times and 

 by different people. One part of the law is first ascertained, 

 afterwards another part: one set of observations teaches us 

 that the law holds good under some conditions, another 

 that it holds good under other conditions, by combining which 

 observations we find that it holds good under conditions much 

 more general, or even universally. The general law, in this 

 case, is literally the sum of all the partial ones; it is the 

 recognition of the same sequence in different sets of instances ; 

 and may, in fact, be regarded as merely one step in the pro- 

 cess of elimination. That tendency of bodies towards one 

 another, which we now call gravity, had at first been observed 

 only on the earth's surface, where it manifested itself only as a 

 tendency of all bodies towards the earth, and might, therefore, 

 be ascribed to a peculiar property of the earth itself: one of 

 the circumstances, namely, the proximity of the earth, had 

 not been eliminated. To eliminate this circumstance required 

 a fresh set of instances in other parts of the universe : these 

 we could not ourselves create ; and though nature had created 

 them for us, we were placed in very unfavourable circum- 

 stances for observing them. To make these observations, fell 

 naturally to the lot of a different set of persons from those 



