Objects of mere rational Ideas, which for theoretical knowledge cannot be presented in any possible experience, are so far not cognisable things, and consequently in respect of them we can form no opinion; for to form an opinion a priori is absurd in itself and the straight road to mere chimeras. Either then our proposition is certain a priori or it contains nothing for belief. Therefore things of opinion are always Objects of an empirical cognition at least possible in itself (objects of the world of sense); but, which, on account merely of the [low] degree of this faculty that we possess, is for us impossible.

Thus the ether of the new physicists, an elastic fluid pervading all other matter (mingled intimately with it) is a mere thing of opinion, yet is