HYPOTHESIS 125 



varies directly as the product of M; and M 2 , their masses, and 

 inversely as the square of their distance (D 2 ) asunder 



But, beyond all this lies the question as to what is the de 

 termining cause of those motions. And as to this, we are indeed 

 free to assume that it is a certain natural property, an active 

 power or force (228), in the bodies themselves, in virtue of which 

 they determine or bring about these motions in this precise 

 manner and measure. But beyond the mere fact, which the 

 principle of causality forces us to admit, that there is in the 

 bodies which constitute the visible universe some adequate cause 

 of those motions, some force that produces them, we know as yet 

 practically nothing. What is the nature of that force? How 

 does it determine and bring about those motions whose magni 

 tude we can accurately estimate by an already verified law? 

 What sort is that influence? How is it exerted through space, 

 independently, as it would appear, of intervening bodies? We 

 describe it as &quot; attraction,&quot; but what idea does this term convey 

 to our minds ? Thus, as to the nature of the cause in question, 

 as to what kind the force of gravity is, and how it acts, we are 

 still largely in the dark. Here, then, is a field for further 

 hypotheses, hypotheses of cause, whose purport will be to explain 

 the known fact of gravitation. Various hypotheses have been 

 framed at different times to connect this fact with the hypotheti 

 cal all-pervading ether, and with the fundamental constitution of 

 matter. 1 So far, however, these are conjectures hazarded to help 

 our imagination in picturing the phenomenon to ourselves, rather 

 than possible explanations of it. Hypothesis, as we have de 

 fined it (226), is essentially explanatory : a supposition that does 

 not offer an explanation of phenomena, but merely aids us in 

 conceiving and describing them, would appear to fall outside our 

 definition. Every hypothesis of cause, i.e. every supposition of 

 some definite antecedent (or group of antecedents) as being the 

 real, actual cause of the phenomenon in question, is necessarily 

 explanatory : it offers provisionally an explanation of the 

 phenomenon. Hypotheses of law, on the other hand, in so far as 

 they merely describe with mathematical exactness the manner 

 in which phenomena occur, are rather descriptive than explanatory ; 

 but nevertheless, inasmuch as a correct quantitative estimate of 



1 Cf. NYS, Cosmologie, p. 125. LESAGE, The Unseen Universe, 140. PICTET, 

 &tudc critique du materialisme et du spiritualisme, p. 239. 



