854. Mr. J. S. Stuart Glennie on tne 
ton’s opposition to which is implied in his Regula Tertia Philo- 
sophands. 
No less clear is it that the postulate of two different kinds of 
matter, one with the qualities of inertia and elasticity, the other 
without the second of these qualities, is opposed by the very 
terms, not only of the third rule, “ Qualitates corporum que 
intendi et remitti nequeunt, queeque corporibus omnibus compe- 
tunt in quibus experimenta instituere licet, pro qualitatibus cor- 
porum universorum habendz sunt,” but by the terms of the 
first rule also, “causas rerum naturalium non plures admitti 
debere, quam que et vere sint et earum phenomenis expli- 
candis sufficiant.” 
22. Consider, secondly, how such hypotheses are judged by 
the modern principles of Metaphysics. For it is evident that 
the theories of every science must ultimately be judged by the 
results of a science, Téyvn Texvov Kal eTLoTHUN ETLOTNLOV, 
which, defining the conditions of knowledge, gives canons for 
the criticism of hypotheses. As this is no place for a metaphy- 
sical discussion, let it suffice to say that the theory of the rela- 
tivity of cognition seems to justify the enouncement of this canon 
as a test of theories put forward as scientific. A scientific (phy- 
sical) theory is founded on postulates of Relations, not on postu- 
lates of absolutely existing Entities. According to this rule it is 
evident that if, for instance, a theory requires an atom of a cer- 
tain size or hardness, it can only be granted where it will stand 
as an expression of the relation between the forces distinguished 
at that point as internal and external; so if a certain elasticity, 
rotatory, or other motion of a body is required, the theory must 
take that elasticity or rotatory motion, not as an absolute pro- 
perty, but along with those relative conditions of other bodies 
which determine such elasticity or motion. 
23. Without advancing any other defence, this canon may be 
justified by the consequence of its neglect. For a theory founded 
on postulates of absolute qualities—entities—must necessarily 
reason in a circle, accounting for phenomena by the same phe- 
nomena already assumed as ultimate. 
Thus, though Professor Challis says that it would be contrary 
to principle “to ascribe to an atom the property of elasticity, 
because, from what we know of this property by experience, it is 
quantitative, and being most probably dependent on an aggrega- 
tion of atoms, may admit of explanation by a complete theory of 
molecular forces*,” he has no hesitation in ascribing elasticity 
to the particles of the ether, which, if anything, are as much 
atoms of matter as the “hard” atoms. But further, as to hard- 
ness, 1s it not the case that, “from what we know of this pro- 
* Phil, Mag, February 1860, p. 89. 
