MATTER 247 



sense-impression ; they even describe it as a " direct 

 object of sense," but from the psychological standpoint 

 force must either be a sense-impression or a group of 

 sense - impressions, for as source or object of sense- 

 impressions it would be purely metaphysical. But as a 

 group of sense-impressions in us, force cannot be that 

 which causes motion in an objective world. As to our 

 muscular appreciation of force, that is a point to which we 

 shall find occasion to return later. We ought not, how- 

 ever, to lay much stress on these authors' remarks as to 

 matter, for they expressly tell us that what matter is will 

 be further discussed in another chapter of their work. 

 Unfortunately, this portion of their great treatise has 

 never been published, although they wrote the above 

 remarks more than twenty-five years ago. Perhaps, had 

 they returned to the subject, they would have recognised 

 that, if the word matter had not appeared more frequently 

 in their text than it does in their index, their volumes 

 would have lost not an iota of their inestimable value to 

 the physicist. 



One of the two authors of the Treatise on Natural 

 Philosophy has, however, published a separate work, 

 entitled. The Properties of Matter. On pp. 12-13 of that 

 work we have no less than nine, and on pp. 287-91 we 

 have no less than twenty-five definitions or descriptions of 

 matter, yet so far from matter being rendered intelligible 

 by all these statements with regard to it. Professor Tait 

 himself writes : — 



" We do not know, and are probably incapable of dis- 

 covering, what matter is." And again : " The discovery of 

 the ultimate nature of matter is probably beyond the range 

 of Jiuman intelligence." 



Now these statements mark a considerable advance on 

 the standpoint of the Treatise on Natural Philosophy. 

 They will at least suggest to the reader that it is no 

 mere whim on my part to question the right of matter to 

 appear at all in scientific treatises. When one author 

 tells us it is a primary conception of the human mind, and 

 another that it is probably beyond the range of human 



