738 THE PRINCIPLES OF SCIENCE. [CHAP. 



bodies possessing inertia are found to gravitate. But it 

 would be no reproach to our scientific method, if something 

 were ultimately discovered to possess gravity without 

 inertia. Strictly defined and correctly interpreted, the law 

 itself would acknowledge the possibility; for with the 

 statement of every law we ought properly to join an esti- 

 mate of the number of instances in which it has been 

 observed to hold true, and the probability thence calcu- 

 lated, that it will hold true in the next case. Now, as we 

 found (p. 259), no finite number of instances can warrant 

 us in expecting with certainty that the next instance will 

 be of like nature ; in the formulas yielded by the inverse 

 method of probabilities a unit always appears to represent 

 the probability that our inference will be mistaken. I 

 demur to the assumption that there is any necessary truth 

 even in such fundamental laws of nature as the Indestruc- 

 tibility of Matter, the Conservation of Energy, or the Laws 

 of Motion. Certain it is that men of science have recog- 

 nised the conceivability of other laws, and even investigated 

 their mathematical consequences. Airy investigated the 

 mathematical conditions of a perpetual motion (p. 223), 

 and Laplace and Newton discussed imaginary laws of forces 

 inconsistent with those observed to operate in the universe 

 (pp. 642, 706). 



The laws of nature, as I venture to regard them, are 

 simply general propositions concerning the correlation of 

 properties which have been observed to hold true of 

 bodies hitherto observed. On the assumption that our 

 experience is of adequate extent, and that no arbitrary 

 interference takes place, we are then able to assign the 

 probability, always less than certainty, that the next 

 object of the same apparent nature will conform to the 

 same laws. 



Infiniteness of the Universe. 



We may safely accept as a satisfactory scientific hypo- 

 thesis the doctrine so grandly put forth by Laplace, who 

 asserted that a perfect knowledge of the universe, as it 

 existed at any given moment, would give a perfect know- 

 ledge of what was to happen thenceforth and for ever 

 after. Scientific inference is impossible, unless we may 



