An _aesthetical Idea_ cannot become a cognition, because it is an _intuition_ (of the Imagination) for which an adequate concept can never be found. A _rational Idea_ can never become a cognition, because it involves a concept (of the supersensible), corresponding to which an intuition can never be given. Now I believe we might call the aesthetical Idea an _inexponible_ representation of the Imagination, and a rational Idea an _indemonstrable_ concept of Reason. It is assumed of both that they are not generated without grounds, but (according to the above explanation of an Idea in general) in conformity with certain principles of the cognitive faculties to which they belong (subjective principles in the one case, objective in the other). _Concepts of the Understanding_ must, as such, always be demonstrable [if by demonstration we understand, as in anatomy, merely _presentation_];[101] _i.e._ the object corresponding to them must always be capable of being given in intuition (pure or empirical); for thus alone could they become cognitions. The concept of _magnitude_ can be given _a priori_ in the intuition of space, _e.g._ of a right line, etc.; the concept of _cause_ in impenetrability, in the collision of bodies, etc. Consequently both can be authenticated by means of an empirical intuition, _i.e._ the thought of them can be proved (demonstrated, verified) by an example; and this must be possible, for otherwise we should not be certain that the concept was not empty, _i.e._ devoid of any _Object_. In Logic we ordinarily use the expressions demonstrable or indemonstrable only in respect of _propositions_, but these might be better designated by the titles respectively of _mediately and immediately certain_ propositions; for pure Philosophy has also propositions of both kinds, _i.e._ true propositions, some of which are susceptible of proof and others not. It can, as philosophy, prove them on _a priori_ grounds, but it cannot demonstrate them; unless we wish to depart entirely from the proper meaning of this word, according to which _to demonstrate_ (_ostendere_, _exhibere_) is equivalent to presenting a concept in intuition (whether in proof or merely in definition). If the intuition is _a priori_ this is called construction; but if it is empirical, then the Object is displayed by means of which objective reality is assured to the concept. Thus we say of an anatomist that he demonstrates the human eye, if by a dissection of this organ he makes intuitively evident the concept which he has previously treated discursively.