12 NATURE AND LIFE. 
compare its primal activities with the only activity of 
which we have direct knowledge and intuition, that is to 
say, with that admirable spring of will,so prompt and sure, 
which permits us every moment to create and also to guide 
motion. 
Motion may serve to measure force, but not to explain 
it. Itisas subordinate to the latter as speech is to thought. 
In truth, motion is nothing else than the series of successive 
positions of a body in different points of space. Force, on 
the other hand, is the tendency, the tension, which deter- 
mines the body to pass continually from one to the other 
of these points; that is to say, the power by which this 
body, considered at any instant in its course, differs from 
the identical body atrest. Evidently this something which 
is in one of these two bodies and is not in the other, this 
something that mathematicians call the quantity of motion, 
which is transformed, on a sudden stoppage of motion, into 
a certain quantity of heat, this something is a reality, dis- 
tinct from the trajectory itself; and yet nothing, absolutely 
nothing, outside of the inner revelation of our soul, gives 
us the means of understanding what this initial cause of 
the motive forces may be. The distinguished founder of 
the mechanical theory of heat, Robert Mayer, defines force 
to be “ whatever may be converted into motion.” There 
is no formula that so well expresses the fact of the inde- 
pendence and preéminence of force, or so completely in- 
cludes the assertion of the essential reality of a cause pre- 
existing motion. The idea of force is one of those ele- 
mentary forms of thought from which we cannot escape, 
because it is the necessary conclusion, the fixed and unde- 
stroyable residue from the analysis of the world in the 
alembie of our minds. The soul does not find it out by 
discursive reasoning, nor prove it to itself by way of theo- 
rem or experiment; it knows it, it clings to it by natural 
and unconquerable affinity. We must say of force what 
