178 CORRELATION OF PHYSICAL FORCES. 



as, when we weigh it, our evidence is the force of attraction ; 

 so, again, our evidence of force is the matter it acts upon. 

 Thus, matter and force are correlates, in the strictest sense of 

 the word ; the conception of the existence of the one involves 

 the conception of the existence of the other : the quantity of 

 matter again, and the degree of force, involve conceptions of 

 space and time. But to follow out these abstract relations 

 would lead me too far into the alluring paths of metaphysical 

 speculation. 



That the theoretical portions of this Essay are open to 

 objection I am fully conscious. I cannot, however, but think 

 that the fair way to test a theory is to compare it with other 

 theories, and to see whether, upon the whole, the balance of 

 probability is in its favour. Were a theory open to no objec- 

 tion it would cease to be a theory, and would be called a law ; 

 and were we not to theorise, or to take generalised views of 

 natural phenomena until those generalisations were sure and 

 unobjectionable, science would be lost in a complex mass of 

 unconnected observations, which would probably never disen- 

 tangle themselves. Excess on either side is to be avoided ; 

 although we may often err on the side of hasty generalisation, 

 we may equally err on the side of mere elaborate collection 

 of observations, which, though sometimes leading to a valu- 

 able result, yet, when cumulated without a connecting link, 

 frequently occasion a costly waste of time, and leave the 

 subject to which they refer in greater obscurity than that in 

 which it was involved at their commencement. 



Collections of facts differ in importance, as do theories : 

 the former, in many instances, derive their value from their 

 capability of generalisation ; while, conversely, theories are 

 valuable as methods of co-ordinating given series of facts, and 

 more valuable in proportion as they require fewer exceptions 

 and fewer postulates. Facts may sometimes be as well ex- 

 plained by one view as by another, but without a theory they 

 are unintelligible and incommunicable. Let us use our utmost 

 effort to communicate a fact without using the language of 

 theory, and we fail ; theory is involved in all our expressions ; 

 the knowledge of bygone times is imported into succeeding 

 times by terms involving theoretic conceptions. As the know- 



