In his Critique of Pure Reason, Kant famously argues for what he calls the "transcendental ideality" of space. A key step in Kant"s argument is his attempted proof in the Transcendental Aesthetic that our most "original" representation of space must be an intuition rather than a concept, and moreover, must be one that is pure, insofar as it must be in the mind a priori, prior to all actual "empirical" (sensation-involving) intuitions of external objects, what Kant calls "outer intuitions". Kant thinks this intuition of space must be present (or "occur") in the mind a priori since spatial representation is universally and necessarily involved in all of our outer intuitions. Kant then goes on to argue (briefly in the first Critique but then at length in the Prolegomena) that accepting his account of the pure intuition of space is also necessary in order to make sense of how it is possible that we could come to have the a priori cognition of space in pure geometry that Kant, along with most of his contemporaries, assumes that we possess.