THE LAW OF SUBSTANCE. 231 



and that is my own mind." This audacious 

 spiritualism seems to us to rest on an erroneous 

 inference from Kant's correct critical theory, that we 

 can know the outer world only in the phenomenal 

 aspect which is accessible to our human organs of 

 thought — the brain and the organs of sense. If by 

 those means we can attain only an imperfect and 

 limited knowledge of the material world, that is no 

 reason for denying its existence altogether. In my 

 opinion, the existence of ether is as certain as that of 

 ponderable matter — as certain as my own existence, 

 as I reflect and write on it. As we assure ourselves 

 of the existence of ponderable matter by its mass and 

 weight, by chemical and mechanical experiments, so 

 we prove that of ether by the experiences and experi- 

 ments of optics and electricity. 



Although, however, the existence of ether is now 

 regarded as a positive fact by nearly all physicists, 

 and although many effects of this remarkable 

 substance are familiar to us through an extensive 

 experience, especially in the way of optical and 

 electrical experiments, yet we are still far from being 

 clear and confident as to its real character. The 

 views of the most eminent physicists, who have made 

 a special study of it, are extremely divergent; they 

 frequently contradict each other on the most important 

 points. One is, therefore, free to choose among the 

 contradictory hypotheses according to one's know- 

 ledge and judgment. I will put in the following 

 eight theses the view which has approved itself to 

 me after mature reflection on the subject, though I 

 am no expert in this department. 



I. — Ether fills the whole of space, in so far as it is 

 not occupied by ponderable matter, as a continuous 



