287 
contrary, the same remark applies as before. In a system 
under attractive forces, the motion, like the force, increases 
when the system contracts, and lessens when it expands. With 
a system of repulsive actions it is the reverse. But in either 
case, or a combination of both, the motion is not constant, but 
may increase continually, from a state of absolute rest to one 
of immense and ceaseless activity. 
The maxim then, that Force is constant, indestructible, and 
unvarying, whether the term be taken in its strict and proper, 
or in its less proper and secondary meaning, is quite untrue. 
It varies in amount continually, with every change in the 
system to which the forces and motions belong. Let us see 
whether Mr. Spencer can throw any light on this great 
difficulty. How does he show that it is a self-evident, a priori 
truth, of which the opposite is inconceivable ? 
The proof he offers consists of two elements. First, we cannot 
measure and compare forces without assuming a unit of force. 
Now this unit is arbitrary. We can never prove by experience 
that it does not vary. Thus an a posteriori proof of the constancy 
of Force is impossible. Therefore, since it is certainly true, and 
cannot be proved by any amount of experience, it must of 
course be an a priori truth (F. P., pp. 185-188). 
The desired conclusion is thus reached with surprising 
facility. And plainly there is no falsehood which may not be 
promoted into an a priori necessary truth, in the same easy 
way. First, assume it to be true. Next, show that no experi- 
ence has proved it, or can prove it. It will then result at once 
that it must be an a priori truth. 
The second part of the proof is equally simple. The equality 
of action and reaction is Newton’s third law, and assumed in 
nearly all dynamical reasoning. But to assert this is to assert 
that force is persistent. 
Now, first, Newton gives four pages, after stating the 
law, to prove it by various experiments. This is a strange 
warrant for the doctrine that it is true a priori, and that 
the converse or negation of it is inconceivable. Next, in a 
recent work on molecular mechanics, a denial of this principle 
is assumed in the main hypothesis, and the results of this 
unthinkable idea are thought out, through nearly three hun- 
dred pages of calculation and reasoning. But besides this 
double disproof of Mr. Spencer’s assertion, the Persistence or 
Constancy of Force, and the equality of Action and Enaction, are 
wholly distinct and almost independent in their meaning. Let 
us take the simplest case. Let two attracting atoms fall to- 
wards each other in a straight line. Their action and reaction 
x 2 
