cease to be an axiom, but would be actually untrue. The sup- 
position, therefore, that the force is dissipated in the cases 
which we have been considering, so far from favouring Mr. 
Spencer’s view, would be fatal to it. It would assume that 
the attractions of the sun and of planets may alter, whereas 
Mr. Spencer’s position is that they cannot alter, for that if 
this were supposed possible all dynamical calculations and all 
astronomical predictions would be uncertain. 
30. But it may be asked — Why, then, is the constancy of 
the unit of force so confidently assumed, if it be true that it is 
not an axiom, and yet that it cannot be proved either a 'priori 
or by experiment ? The answer is, that there are various 
kinds and degrees of proof; and there are degrees of proba- 
bility which amount practically to certainty. Most of us must 
remember the instance given by Bishop Butler of this very 
high degree of probability, viz., the confident expectation 
entertained by all that the sun will rise to-morrow. No proof 
of this can be given which would lead to absolute certainty, 
and yet all our arrangements for the future are based on the 
assumption that each day will be like those which precede and 
follow it. Mr. Spencer would say that this necessarily follows 
from the persistence of force, which causes the earth to revolve 
uniformly on its axis. But as the persistence of force is the 
principle actually under discussion, we cannot accept it as 
demonstrating to an absolute certainty the recurrence of a 
terrestrial day. W e are practically certain of such recurrence, 
but we have not the certainty of demonstration. Now, I 
believe that we have a similar kind of certainty of the per- 
sistence of force, derived from our experience, which enables 
us to assume for practical use the consistency of the unit of 
force, and to believe that it neither has varied nor will vary 
in the course of any time with which we have to do, unless 
it should at any time seem good to the great Creator of all 
things to alter or annihilate it. And this I believe for the 
following reasons : — 
31. Force is known in dynamical reasoning simply as a 
commencement or change of velocity, the mass remaining the 
same. Metaphysically, we believe that every change has a 
cause; and, therefore, that when the velocity of a moving body 
commences or changes, there must be some cause for the 
change, and to this unknown cause we give the name “ force.” 
But this cause does not enter into the mathematical process. 
All that is there taken account of is the velocity, or change of 
velocity, produced in a given time. Now, since velocity is a 
function of time and space, and force is a function of velocity 
and time, the elements, and the only elements, whereby we 
