26 On Some Processes of Scientific Reasoning. 



2. The effort to move it. 



3. Its motion. 



Any one of these three may be isolated from the others. 

 If I am paralysed, I may will to move my arm, but am 

 incapable of exerting any effort to move it. If my legs are 

 tied down, and the soles of my feet tickled, there will, quite 

 independently of, and even in opposition to my will, be an 

 effort to move my leg, which is not followed by sensible 

 motion. If somebody else takes hold of my arm and pulls 

 it, we have motion without being conscious of volition or 

 effort. 



Motion, however, is only one of the effects which effort 

 can produce ; there are others, e.g., if I press my two hands 

 together, I have effort producing pressure. Now, these 

 effects which conscious effort can produce may be produced 

 otherwise, as by tying a weight to my arm. Force is the 

 name for anything which can produce the effects Effort pro- 

 duces ; in fact Effort is a species of Force, though it does not 

 follow that all Force is Effort. We may speak of a weight 

 as a Force, or, as is sometimes done, we may speak of the 

 weight as having a Force inherent in it. However we may 

 picture Force to our imagination, it is a metempirical 

 conception. All we can know of Force by experience is the 

 phenomenal effects it produces. Yet, although a metem- 

 pirical conception, the idea of Force is a most valuable one, 

 and enables us to describe phenomenon much more clearly 

 and concisely than could be done without employing it. 



Having attempted an exposition of the nature of our con- 

 ceptions of Matter and Force, I now proceed to show how 

 Ideal Construction is employed in Dynamics. 



Dynamics is generally divided into four parts — Dynamics 

 of a particle, of a rigid body, of a fluid, and of a gas. Into 

 each of these divisions Ideal Construction enters. There are 

 no objects in nature which fulfil the definitions of a 

 particle, rigid body, fluid, or gas. Yet there are many 

 objects which, to our senses, differ so little from these Ideal 

 conceptions, that the conclusion of Abstract Dynamics may be 

 applied to them without practical error. We may also 

 notice the Ideal conceptions of perfectly smooth bodies, 

 flexible strings, &c. In dealing with the subject of Impact, 

 an Ideal construction is employed, viz., the Idea of bodies 

 which after coming into contact with each other, immediately 

 rebound. As a matter of fact, an interval of time always 



