Reality of Force. 393 
to us in Nature, namely Matter and Energy; and his argument 
is based on the fact that both of these are subject to the Law 
of Conservation. But I have elsewhere shown at length* 
that the whole of the recognised laws of Mechanics, including 
the Conservation of Energy and Matter, flow directly from the 
three Laws of Motion (if not from more general principles still), 
if we take as our definition of Matter that it is a " collection 
of centres of force distributed in space, and acting upon each 
other according to laws which do not vary with time, but do 
vary with distance." I have also shown f that the second 
principle — the Conservation of Energy — does not hold in any 
cases where the forces are not of the above character. Hence 
instead of the four fundamental realities Space, Time, Matter, 
and Energy, we need only three — Space, Time, and Force; 
and from these the mechanical universe, as we know it, can be 
constructed. But it will not be contended that we know 
anything of Energy as an independent objective reality, except 
what is revealed to us in the study of Mechanics; in fact, its 
existence was never even suspected until the modern develop- 
ment of that study had begun. Hence it appears that all the 
facts forthcoming to prove its independent existence can be 
perfectly accounted for apart from that hypothesis; and that 
being so, the evidence in favour of the hypothesis sinks abso- 
lutely to zero. But that for which there is no evidence is not 
to be believed. 
I will here conclude this paper, perhaps already too long. 
If any illustrations used are not of a kind ordinarily adduced 
in such discussions, it is Prof. Tait's old-school doctor and 
his " subjects " who must be my excuse. For the paper itself 
I do not make any excuse, because I am convinced that the 
new views promulgated by Prof. Tait, and by others, on 
the foundations of Mechanics are doing very serious harm, 
especially among those who approach the subject from the 
practical side. It is not that they are led to inquire more 
closely into these fundamental principles, and the evidence for 
them — that would be a useful result ; but they are led to think 
that there is no real ground of truth in any of them — that they 
are mere convenient working hypotheses, which may be left 
to contradict and stultify each other just as may happen. When 
this belief is fully accepted, the era of fruitful progress in 
Physical Science will be at an end. 
* < The Student's Mechanics (Charles Griffin & Co., 1883). 
t Phil. Mag. 1883, p. 35. 
