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Froceedings oj the Foyal Society 
possess inertia or the feeling of ponderosity or immobility, the 
difficulty should vanish when it is kept in mind that all our percep- 
tions of force are only relative, not absolute. If our living bodies, 
then, and the substance of all external objects, are of the same 
immaterial nature, we should not be surprised, but should rather 
expect, that the inertia of external objects should appear in propor- 
tion to their masses, and that it should also have a relation to the 
strength as well as the mass of our percipient bodies, all being com- 
posed of the same immaterial substance. 
The great difficulty felt by those metaphysicians who believe in 
matter has always been regarding this inert thing, matter. Take 
away, say they, the qualities of colour, heat and cold, resistance or 
solidity, from any object, and an inert something remains to puzzle 
us. The author, on the contrary, holds that resistance or solidity 
constitutes matter. The difficulty felt by metaphysicians is thus 
avoided — namely, the necessity of conceiving a thing to exist with- 
out qualities. 
There is another difficulty which besets the believer in matter. 
The human mind has always felt a difficulty — an apparent incon- 
gruity, almost approaching to a feeling of impossibility — when it 
conceives of a Being, whose essence is spiritual, creating a thing of 
a different essence from Himself, which matter is conceived to be. 
The ancient philosophers of G-reece, feeling this, declared that mat- 
ter was uncreated, and eternal. Spinosa, one of the acutest minds, 
felt also the same difficulty, and in his Ethics he lays it down as an 
axiom of reason that the knowledge of an effect (the world, for 
instance) depends on the knowledge of the cause, and things that 
have nothing in common with each other (matter and spirit) cannot 
be understood by means of each other.” Hence the one cannot be 
the cause of the other. We state this principally to show how 
extensively the difficulty has been felt of conceiving the existence 
of two different essences in nature. 
Kecent discoveries have established that heat is mechanical force, 
the two being mutually convertible without loss. The attraction 
of gravity and chemical attractions and repulsions are all the same 
physical force, and the entire external world is nothing but a 
manifestation of it,— a simple and grand conception, and one which 
enters the domain alike of physics, of speculative philosophy, and 
