10 
will be proper to remark that hypotheses proposed for physical 
inquiry are adapted to meet every demand that may be 
legitimately made upon them, if they are expressed in terms 
rendered intelligible by sensation and antecedent experience; 
and if they consist only of definitions and postulates which 
involve no variable elements, and on that account are suitable 
for being made foundations of theoretical calculation. Pre- 
suming, as I think I may for the reasons already given, that 
the adopted three hypotheses do in fact fulfil those conditions, 
I am entitled to disregard any mere expression of disapproval 
of them, whether wholly or in part, inasmuch as their claim to 
acceptance is to be tested, and can only be established, by 
comparison of results obtained from them by mathematical 
reasoning with certified facts. Any arguments, however, 
bearing upon the validity of such reasoning, I am bound to 
take notice of, and, to the best of my ability, shall endeavour 
to answer. 
16. It will be also worth while to advert here to a mode of 
philosophy advocated in the present day, which is directly 
opposed to the rules of philosophizing laid down in Newton’s 
Principia. It appears that some of my contemporary physicists 
absolutely refuse to accept the method of conducting physical 
inquiry by means of a priori hypotheses, although (as has been 
argued in secs. 6-8), Newton adopted this process in his 
theory of universal gravitation ; and also gave rules for applying 
an analogous method to account theoretically for the laws 
which govern the various kinds of relation between matter and 
force. This opposition to the Newtonian a priori principles of 
philosophy comes mainly from the advocates of views such as 
those which are developed in the work entitled The Unseen 
Universe. I propose, therefore, as contributing to the pur- 
pose of this essay, to state briefly what I conceive to be the 
origin and character of those views, and why they are incom- 
patible with the Newtonian philosophy. 
17. The principles of physical philosophy as respects the 
ultimate qualities of matter and force, which were so well pro- 
pounded at the epoch of Locke and Newton, were in a short 
time set aside by the admission of hypotheses not conformable 
with the Newtonian rules of philosophizing. In particular, it 
was assumed that two portions of matter in presence of each 
other mutually attract, in virtue of intrinsic force resident in 
an unintelligible manner in each, and acting in an unintelligible 
manner through the space between them. Newton distinctly 
repudiated this hypothesis. It was so framed that while it 
allowed of ascertaining the law of the mutual action as depend- 
ing on the distance between the bodies, it precluded all inquiry 
