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18. Bat when we proceed to apply these logical pi^ocesses 
to physical relations, we tread on different ground altogether. 
Our senses train us to form certain conceptions as to material 
substance, and its motion or other relations to space and time. 
We conceive of matter as something occupying space, so that 
no portion of space, the whole of which is filled by one portion 
of matter, can at the same time contain any other. We further 
conceive of it as inert, that is, absolutely incapable of altering 
its own conditions. These two conceptions direct us to 
certain fundamental laws of the motion of material bodies ; 
the laws being, as far as we can judge, necessary consequences 
of our primary conceptions. At the same time, it must be 
remembered that the confidence which the mind now feels in 
these laws of motion was very slowly attained, and has arisen 
from the fact that the results of almost innumerable observa- 
tions coincide with the results of those calculations which are 
made on the assumption of their truth and their universality. 
The senses, indeed, are both the origin and the verification 
of physical science, even in its most exact form. And this is 
even more manifestly true in reference to that extension of 
these laws which has been made in modern times, and which 
is known as the conservation of energy. 
19. There is, however, another idea which our senses also 
suggest in regard to material things, the idea of force as that 
which causes either motion or a resistance to motion. In 
dynamics, or the science of foi’ce, this is measured by the 
velocity it would generate, if acting uniformly on a unit of 
mass through a unit of time ; and if this effect of the 
force be known, the effect when the conditions are altered to 
other known conditions may be determined. But while 
science can thus investigate and compare the several effects 
of that which is called force, it teaches us nothing whatever, 
except in one particular case, of the causation itself. The 
one case in which the causation of motion is the necessary 
consequence of our original conceptions of material substances 
is when two or more incompressible bodies, having different 
motions, come into collision; and then the cause of the result- 
ing motions is known to be the antecedent motions, the effects 
being determined by those laws of motion which are essential 
to matter, as we conceive it. But in every other case of the 
causation of motion the word force is merely the disguise of 
our ignorance ; it stands for the unknown cause of certain 
effects. But if no reason for the causation can be given by 
science, this means that science is unable to determine the 
law of the force as a necessary truth ; and, therefore, the aim 
of science is, and must be if the domain of reason is to be 
VOL. XII. z 
