1903.] STONEY—UNIVERSE OF REAL EXISTENCES, 129 
justification of the physical hypothesis is its utility, not its truth— 
its incomparable efficiency as a means of investigating nature. 
This is a matter about which it is far better, although it cannot be 
said to be essential, that students of Physics should make no mis- 
take. 
It is obvious that causation, in the full sense of that term, can 
operate only-between the real existences of the autic universe, and 
that everything else that appears to us to take place isa consequence 
of what occurs there. In fact, efficient cause and autic cause are 
synonymous terms. 
Nevertheless, it is convenient and quite legitimate for scientific 
-men to speak of ‘ physical’ causes, meaning thereby what they have 
to treat as causes when engaged in carrying on an investigation 
under the Physical Hypothesis. 
_A very useful scaffolding which helps us in building up our inves- 
tigation is the introduction of forces between the physical cause 
(which is always the vicinity of some natural object) and the effect 
to be attributed to it under the physical hypothesis. We are thus 
enabled to speak of the acceleration of a stone in its fall towards 
the earth, either as being due to the neighborhood of the earth, or 
as being caused by a force of gravitation which acts on it, which 
force is, in its turn, regarded as brought into existence by the prox- 
imity of the earth to the stone. The introduction of this piece of 
intermediate scaffolding is found to be of service— 
1. Because the force can be represented by a line whose length 
accurately represents the intensity, and whose direction ac- 
curately represents the direction, of the effect upon the stone 
of the vicinity of the earth ; 
2. Because the same effect upon the stone might have been 
due to other physical causes, as, for example, to a spring 
urging it forwards; in which case the same piece of scaf- 
folding, a force represented by the same line in the same 
position, would occupy its place between the cause and the 
effect ; and 
3. Because the effect might have been different, while the 
physical cause remained the same: thus, if the stone lay on 
the ground, what the vicinity of the earth would have occa- 
sioned is stress between the stone and the ground. 
Accordingly, by referring effects in nature to the operation of 
