302 Scientific Proceedings, Royal Dublin Society. 
objects, which are what we see and feel, are supposed to accom- 
pany them in their wanderings by reason of the way in which 
they, the substances, act (usually through intermediate material 
agencies) upon our organs of sense.’ 
This is the usual point of view: but more careful thinkers will 
do well? to eschew this somewhat convenient, but by no means 
necessary, encumbrance upon the unadulterated process of physical 
investigation which treats the sensible objects themselves, the bare 
syutheta of perceptions and ultra-perceptions, as though they were 
what brought about the changes that occur in nature; and which 
exclusively occupies itself in tracing out the laws that must, under 
this hypothesis, be in operation in order that the effects may be 
what they are. 
Another and a very useful scaffolding which helps us in build- 
ing up our investigation, 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 
neighbourhood of the earth, or as being caused by a force of gra- 
vitation which acts on it, which force is, in its turn, regarded as 
brought into existence by the proximity of the earth to the stone. 
The introduction of this piece of intermediate scaffolding is of 
great service— 
1. Because the force can be represented by a line whose 
length accurately represents the intensity, and whose direction 
accurately 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 forward, in which case the same piece of scaffolding, a force 
1 The author has endeavoured to trace out what it is that really occurs in all such 
cases. See his paper ‘¢ On the Relation between Natural Science and Ontology ”’ in 
the Scientific Proceedings of the Royal Dublin Society, vol. vi. (1890), p. 475. 
? This course is much to be preferred, because it effectually avoids the risk of throw= 
ing dust in our own eyes. The justification of the Physical Hypothesis is its utility, 
not its truth—its incomparable efficiency as a means of investigating nature ; and it is 
better, though not essential, that students of Physics should make no mistake about a 
matter of this kind. 
