THOMSON AND TAIT S NATURAL PHILOSOPHY. 779 



metaphysician, but the naturalist may be content to know matter as that which 

 can be perceived by the senses, or as that which can be acted upon by, or can 

 exert, force." 



The authors proceed to throw out a hint about Force being a direct 

 object of sense, and after telling us that the question What is matter? will 

 be discussed in a future volume, in which also the Subjectivity of Force will 

 be considered, they retire to watch the effect of the definition they have thrown 

 into the camp of the naturalists. 



Now all this seems to us very much out of place in a treatise on 

 Dynamics. We have nothing of the kind in treatises on Geometry. We have 

 no disquisitions as to whether it is by touch or by sight that we come to 

 know in what way a triangle differs from a square. We have not even a 

 caution that the diagrams of these figures in the book do not exactly corre- 

 spond with their definitions. Even in kinematics, when our authors speak of 

 the motion of points, lines, surfaces, and solids, though they introduce several 

 modern phrases, the kind of motion they speak of is none other than that 

 which Euclid recognises, when he treats of the generation of figures. 



Why, then, should we have any change of method when we pass on from 

 kinematics to abstract dynamics? Why should we find it more difficult to 

 endow moving figures with mass than to endow stationary figures with motion ? 

 The bodies we deal with in abstract dynamics are just as completely known 

 to us as the figures in Euclid. They have no properties whatever except those 

 which we explicitly assign to them. 



Again, at p. 222, the capacity of the student is called upon to accept 

 the following statement : 



" Matter has an innate power of resisting external influences, so that 

 every body, as far as it can, remains at rest or moves uniformly in a, 

 straight line." 



Is it a fact that " matter" has any power, either innate or acquired, of 

 resisting external influences ? Does not every force which acts on a body 

 always produce exactly that change in the motion of the body by which its 

 value, as a force, is reckoned? Is a cup of tea to be accused of having an 

 innate power of resisting the sweetening influence of sugar, because it persist- 

 ently refuses to turn sweet unless the sugar is actually put into it? 



But suppose we have got rid of this Manichsean doctrine of the innate 

 depravity of matter, whereby it is disabled from yielding to the influence of 



982 



