576 APPENDIX. 



ered as at least probable: support is found for certain theo- 

 logical beliefs. Now, clearly, the relation between the seen 

 and the unseen Universes cannot be the subject of any observa- 

 tion or experiment; since, by the definition of it, one term 

 of the relation is absent. If we have, then, no warrant for 

 asserting a physical axiom save as a generalization of results 

 of experiments if, consequently, where no observation or 

 experiment is possible, reasoning after physical methods can 

 have no place; then there can be no basis for any conclusion 

 respecting the physical relations of the seen and the unseen 

 Universes. Not so, however, concludes Professor Tait. He 

 thinks that while no validity can be claimed for our judg- 

 ments respecting perceived forces, save as experimentally justi- 

 fied, some validity can be claimed for our judgments respect- 

 ing unperceived forces, where no experimental justification 

 is possible. 



The peculiarity thus exhibited in Professor Tait's general 

 thinking, is exhibited also in some of his thinking on those 

 special topics with which he is directly concerned as a Pro- 

 fessor of Physics. An instance was given by Professor Clerk- 

 Maxwell when reviewing, in Nature for July 3, 1879, the new 

 edition (1879) of Thomson and Tait's Treatise on Natural 

 Philosophy. Professor Clerk-Maxwell writes: 



" 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 f " 



And to Professor Clerk-Maxwell's question thus put, the an- 

 swer of one not having a like mental peculiarity with Professor 

 Tait, must surely be No. 



But the most remarkable example of Professor Tait's mode 

 of thought, as exhibited in his own department, is contained 

 in a lecture which he gave at Glasgow when the British Asso- 

 ciation last met there (see Nature September 21, 1876) a 

 lecture given for the purpose of dispelling certain erroneous 

 conceptions of force commonly entertained. Asking how the 

 word force "is to be correctly used" he says: 



" Here we cannot but consult Newton. The sense in which he uses 

 the word * force,' and therefore the sense in which we must continue to 

 use it if we desire to avoid intellectual confusion, will appear clearly 

 from a brief consideration of his simple statement of the laws of motion. 



