138 THE SCIENCE OF LOGIC 



For centuries, the Ptolemaic system of astronomy was accepted as being 

 the only one that could account for the facts. It was only a man of the rare pene 

 tration of Aquinas who could point out that perhaps on some other hypothesis 

 these could be equally well explained : &quot;forte secundum aliquem alium 

 modum nondum ab hominibus comprehensum apparentia circa Stellas sal- 

 vantur &quot; ; 1 and the substitution of the Copernican system, three hundred years 

 afterwards, justified his suspicions. Probably no one at the present day would 

 venture to doubt the truth of the latter system. But there are in current 

 science several &quot; systematic conceptions &quot; of such a character that they can 

 scarcely ever be verified in the stricter sense of being shown to be the only 

 possible explanations of the facts. A few of these will help to illustrate how 

 such conceptions differ from strictly verifiable scientific hypotheses. 



Lord Kelvin s theory that the ultimate atom of matter is a vortex-ring in a 

 perfect liquid, is rejected by Clifford as unscientific because &quot; a perfect liquid is 

 not a known thing but a pure fiction ... a mere mathematical fiction &quot; ; * 

 and, since we can deduce nothing from the absolutely unknown, the hypothesis 

 is unverifiable. 



Laplace s hypothesis that the solar system was at first a rotating nebula 

 from which the planets got detached, and from them in turn their satellites or 

 moons, all of which condensed and cooled down gradually by radiation, and 

 so solidified is not verifiable either by observation or by experiment. It does 

 not bear upon the immediate, but upon the remote, far-distant origin of our 

 earth and solar system, at a time when the conditions of the natural forces at 

 work may have been very different from any with which we are at present 

 familiar. What hope, therefore, can we have of ever proving that these planets 

 could have developed in no other possible way ? It remains, therefore, a mere 

 hypothesis, and may have its function in science ; but it is not a scientific hypo 

 thesis in the strict sense, if by the latter we are to understand a supposition 

 whose truth can be rigorously established, to the exclusion of all alternatives, 

 by that experimental method of which Pasteur has so well said that it &quot; leads 

 to absolute and unanswerable demonstration . . . and deceives none but 

 those who make a bad use of it &quot;. 3 



The same remarks apply with equal force to the conception of an all- 

 pervading ether as a medium for the propagation of light, of radiant heat, of 

 electric and magnetic influence, etc. The existence of some medium is the 

 only alternative to actio in distans, the absolute impossibility of which is not 

 easily demonstrable ; and the nature of the supposed ether, the properties 

 with which it is endowed, are not by any means agreed upon by scientists. 4 

 In those circumstances, no prudent scientist would venture to say that the ether 

 as he conceives it, and the modes of transmission of those various influences as 



1 In Lib. II. de Cash et Mundo, lect. xvii. Cf. Summa Theol. i a , P. Q. 32, a, 

 i, ad 2. DE WULF, Scholasticism Old and New, p. 32. 



2 CLIFFORD, Lectures and Essays, p. 169. apud WELTON, op. cit., ii. pp. 74, 98. 



3 MERCIER, op. cit., pp. 340-1. 



*Cf. The &quot; New Knowledge &quot; and its Limitations, in the Irish Ecclesiastical 

 Record, January, 1910, pp. 23 sqq. the fourth of a series of articles in which are 

 examined in some detail the implications of another of those &quot; systematic concep 

 tions &quot; the electrical theory of matter. Cf. art. on The Philosophy of Energy, ibid., 

 February, 1910, and Some Current Phases of Physical Theories, ibid., April, 1910. 



