﻿Principles of Molecular Physics. 185 



ciples are, in my opinion, not sufficiently probable, contains 

 undoubtedly a great deal of " established truth." 



But " molecular physics," says he, "" cannot be erected, like 

 mathematics, upon a foundation known from the first to be 

 eternally sure, that of self-evident truth " This remark of my 

 learned critic does not show that molecular physics cannot be 

 erected on certain truth, but only that physical science, whether 

 molecular or not, does not deal, like mathematics, with truths 

 which consist of necessary relations, but with truths dependent 

 upon contingent facts. The first are known mainly by reason, 

 the secondly mainly by observation : and no one demurs to 

 physical certainty on the ground that it is not mathematical. 



Professor Norton adds : 



" Mechanical axioms may exist as mere figments of the mind, and 

 have often risen like bubbles in the minds of speculative philosophers, 

 shone with an evanescent splendour, and suddenly burst at the touch 

 of a hard fact." 



I do not deny that axioms may exist as mere figments in the 

 minds of " speculative " philosophers ; but the learned Professor 

 has surely forgotten that all philosophers, who build theories, 

 are " speculative " philosophers. Otherwise how is it possible 

 that he should write a theory of molecular physics, and yet be so 

 hard on speculative philosophers amongst whom his work places 

 him ? 



But Professor Norton apparently wishes to designate as 

 " speculative philosophers " a particular school of philosophers, 

 whose speculations have their origin in a priori reasonings in- 

 stead of natural facts. If so, I may safely reply that such a 

 school (to which he imagines me to belong) has never existed. 

 There has indeed been a school of philosophers, whose physical 

 speculations are now known to have been in most instances 

 illustrious failures; nevertheless, that school notoriously abided 

 by facts; and their failure was caused by misinterpretation of 

 facts, not by any desire of substituting a priori grounds for a 

 posteriori arguments. In what does modern speculation differ 

 from that of the ancients ? We possess, to be sure, a greater 

 number of experimental data; but this advantage (which does 

 not change the nature of scientific speculation) is frequently coun- 

 terbalanced in a great measure by the vagueness and shallow- 

 ness of our philosophical acquirements. Facts and laws are only 

 materials for speculation : something more is needed for building 

 sound physical theories : and this something is not so much the 

 power of imagining new hypotheses, as the art of reasoning and 

 an intimate acquaintance with those speculative principles which 

 apply to the interpretation of facts. Such principles are the 



