203 
16. It is also an essential peculiarity of inertia that it is not 
quantitative ; that is, as Newton has remarked (Regula III.), it is 
not, like gravity, susceptible of any law of variation, lhe same 
matter may be more or less heavy according to circumstances, and 
there may be more or less of inert matter, but under no conditions can 
matter be more or less inert. For these reasons, inertia may be 
said to be an innate quality of bodies ; while the same assertion is 
not true of gravity. To express this quality Newton uses the word 
“ insita.” 
17. It is not necessary to adduce here the usual arguments of 
metaphysicians (those, for instance, of Locke), which show that we 
are cognizant of the other essential qualities of bodies, — magnitude, 
form, and mobility, by means uf the senses of sight and touch. It 
'will suffice to state that in the subsequent reasoning it is taken for 
proved that these fundamental qualities are all cognizable by infor- 
mation given immediately by the senses. Relative to this principle 
Newton makes the assertion, “ This is the foundation of all 
philosophy (Reg- III.). 
18. To carry on the d priori argument respecting the ultimate 
properties of matter, another regulative principle is to be taken into 
account, namely, that these properties are not quantitative. We 
have already seen that the quality of inertia satisfies this condition. 
In order to satisfy it with respect to magnitude and form, we must 
suppose that in any given atom these properties are absolutely 
incapable of variation. The reasons for this second regulative 
principle cannot be fully unfolded till we have had Force as well as 
Matter under consideration. So much, however, as this may be 
said at present : Whatever is quantitative, or variable in a manner 
expressible by numbers, admits of being determined by mathema- 
tical reasoning. Such reasoning is necessarily based on principles, and 
conducts, according to ascertained rules, to quantitatively expressed 
results, or laws. The principles in the present inquiry are ultimate 
physical qualities, or properties ; which, consequently, must be such 
as are not susceptible of quantitative variation. 
19. With respect to the forms of atoms, if Newton’s dictum, 
that “ nature is wont to be simple and consonant with herself’ 
(Reg. HI-), be accepted, we might say at once that all atoms have the 
spherical form. Also “ the tenor of facts of experience,” attention 
to which is another rule laid down by Newton for forming a priori 
conceptions, conducts as follows to the same inference. If we 
agitate water in a vessel in any manner, and then leave it to settle 
into its original state, it will in all respects and for all purposes be 
the same water as before. But the agitation will have altered the 
relative positions of the ultimate parts in an unlimited number of 
