12 
of experiment are shown by mathematical reasoning to be 
consequences of ulterior intelligible principles. The perfec- 
tion of physical science consists in giving reasons for physical 
laws. 
18. In order, farther, to exhibit the antagonism of the 
philosophy of The Unseen Universe to that of Newton, I 
quote as follows from Art. 139, page 107 (1st ed.) : — "After 
inertia, which is not accounted for by any of the hypotheses 
as to the ultimate nature of matter which we have just given, 
the most general property of matter which we recognize is that 
of universal gravitation/'’ This assertion can, I think, be only 
understood as meaning that gravity is a property of matter in 
the same category as inertia ; whereas Newton says, at the end 
of his Third Rule of Philosophy, that he by no means affirms 
gravity to be essential to bodies, that he takes vis inertice to be 
the only intrinsic ("insita") force, and that this force is 
invariable (" iinmutabilis "), whilst, on the contrary, gravity 
diminishes with increase of distance from the earth. (These 
views accord with tho rule I have adopted in sec. 11, of not 
admitting qualities susceptible of variation to be primary, which 
rale, of course, excludes gravitation from the class of primary 
qualities.) It is right, however, to take into consideration that 
although these authors speak of gravitation as a “ property " 
of matter, they fully assent to Newton's dictum respecting the 
unreasonableness of the assumption of action at a distance 
without intermediate agency (U. U., Art. 140, p. 109). But 
the acceptance of Newton's authority in this particular, which 
is hardly consistent with their treatment of his philosophy in 
other respects, is followed by a statement of various supposi- 
tions made to account for gravitation, which appear to be of 
an extremely speculative and arbitrary character. Preference 
seems to be given to the agency of “ ultra-mundane corpuscles, 
in infinite numbers, flying about in all directions with velocities 
enormously great." These corpuscles are supposed to rain 
freely on the interior particles of masses, and by their impacts 
to produce the effect of gravitation. It is a peculiarity of the 
phase of philosophy I am referring to, to substitute for pres- 
sure, as ordinarily understood, the effect of the impacts of an 
immense number of exceedingly minute particles. Thus Pro- 
fessor Tait, in page 324 of the before-cited Lectures, says, 
" One of the results arrived at as to the motion of swarms of 
impinging particles is, that in a mass of hydrogen at ordinary 
temperature and pressure, every particle has, on an average, 
17,700 millions of collisions per second with other particles — 
that is to say, that number of times in every second it has its 
course changed. And yet the particles are moving at a rate 
