232 
follows out the notions of his leader to their utmost limit. 
But we are strongly disposed to think that P-esBey as more 
powerfully influenced by Locke than he was by Hartley per 
haps more influenced, however, by Boscovich than by either. 
hI imagined that this naturalist had demonstrated the mutual 
penetrability of material substances. Light J as ^ 1 1 *8™' 
as a substance, and not, as now, only a kind of motion m 1 1 
molecules of substances. It is not difficult to see how this 
error might lead to the fancy that two particles of mattei 
might be 8 in the same place at the same time. Priestley says, 
“ If the momentum of such a body m motion be sufficient y 
great, Mr. Boscovicb demonstrates that the partic es o any 
body through which it passes will not even be moved out ot 
their Diace bv it.” By “ such a body ” he means one similai 
to light.* Now, it is quite true that “ such a body ” as is not 
“ body ” at all, but merely an agitation of the molecules oi 
that which is illuminated, may pass through anything and no 
displace its particles by taking that place itself. But this s 
wonderfully different from a real body passing through another 
real body without displacing its particles, by occupying in its 
passage their places instead of them. It is on this penetra- 
bility of matter that Priestley founds his idea that spmt is 
material. He says “ I therefore define it (matter) to be a 
substance possessed of the property of extension and of powers 
of attraction and repulsion. And since it has never yet been 
asserted that the powers of sensation and thought are ^com- 
patible with these (solidity and impenetrability only haw i g 
been thought repugnant to them), I therefore maintain that 
we have no reason to suppose that there are in man two sub- 
stances so distinct from each other as have been represented, t 
The fact that the “ affections ” of matter do not necessarily 
displace its particles, looked at under the mistaken notion that 
these “ affections ” were themselves material substances tha, 
could pass through solid bodies, without occupying their space 
in any^ degree, is the (now exploded) foundation of Priestley’s 
whole system of materialism. The plan according to which 
men refuse to hiow whatever does not suit their genraal notions 
had not come into fashion in Priestley’s days. It is the grand 
characteristic feature of the so-called philosophy of our own 
times. A very remarkable instance of it occurs on this \ er} 
subject of materialism in Dr. Davey’s book on the Ganglionic 
Nervous System in the Human Body. { He traces wliat h- 
* See Priestley’s Disquisitions, page 24, edition 1/82. 
f Introduction to Disquisitions, page ii. 
% Dr Davey on the Ganglionic System, pages 69, 80, &c. 
