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ways, and to conceive how such changes should produce no residual 
effect, it seems necessary to suppose the form of each atom to be 
such as to have no special geometrical relation to the positions of 
surrounding atoms. This condition can be satisfied only by the 
spherical form. 
20. By the foregoing considerations we have been conducted to 
the following very definite result: An atom is a very small inert 
sphere of invariable magnitude. But however definite and in- 
telligible this conception of an atom may be, inasmuch as the con- 
siderations leading to it were of an a priori character, we are not 
entitled, without inductive verification, to say that an atom is 
really such. "We may, however, assert that the hypothesis of its 
being such is an appropriate basis of mathematical reasoning, and 
that by comparing results from mathematical reasoning thus 
founded, we can decide whether or not the hypothetical atom is a 
reality. The way in which this has to be done will be indicated 
by the discussion I am about to enter upon relative to the nature 
of Force. The discussion will at the same time take account of 
the property of mobility which is common to all matter. 
21. Force. — From the experiment described in art. 15 we may 
gather that in the production of motion by force, the sense of 
touch tells us that two bodies are concerned, one of which is active, 
and the other relatively passive ; one is the mover and the other 
the moved. As this is a mode of producing motion which is 
intelligible by sensation and experience, according to the philosophy 
I am expounding, there is no mode of producing motion which is 
essentially different from this. I assume that when a body is moved 
by being pushed with the hand, the physical action between the 
body and the hand is precisely the same as when one piece of dead 
matter moves another by pushing against it. And it must be 
admitted that in neither case have we reason to say that the parts 
of one body come into actual contact with the parts of the other. 
But in the case of the personal act there is a felt contact which is 
distinguishable from non-contact- Consequently our philosophy 
necessitates the conclusion that in every case of the production of 
motion (as of an atom by the ether) there is contact, as felt , be- 
tween the moving and the moved body. Of course, if this be so, 
it is a necessary consequence that there is also the consciousness 
of a personal agent. For my part I accept this inference on the 
general principle that it is inconceivable there can be any pro- 
duction or event which is not determined by antecedent will, and 
by the power, in operation, of a conscious agent. But this part of 
the discussion I reserve at present, as it may be more appropriately 
handled with reference to a special metaphysical question that will 
