(1.) 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,[144] an elastic fluid pervading all other matter (mingled intimately with it) is a mere thing of opinion, yet is such that, if our external senses were sharpened to the highest degree, it could be perceived; though it can never be presented in any observation or experiment. To assume [the existence of] rational inhabitants of other planets is a thing of opinion; for if we could come closer to them, which is in itself possible, we should decide by experience whether they did or did not exist; but as we shall never come so near, it remains in the region of opinion. But to hold the opinion that there are in the material universe pure thinking spirits without bodies (viz. if we dismiss as unworthy of our notice certain phenomena which have been published as actual[145]) is to be called poetic fiction. This is no thing of opinion, but a mere Idea which remains over, when we remove from a thinking being everything material, and only leave thought to it. Whether then the latter (which we know only in man, that is, in combination with a body) does survive, we cannot decide. Such a thing is a _sophistical being_ (_ens rationis ratiocinantis_), not a _rational being_ (_ens rationis ratiocinatae_)[146]; of which latter it is possible to show conclusively, the objective reality of its concept; at least for the practical use of Reason, because this which has its peculiar and apodictically certain principles _a priori_, demands (postulates) it.