 The decisive point of this argument is the following: although because of our forms of intuition our particular representations necessarily have spatio-temporal structure, any objects that had that structure independently of our so representing them would at best have such structure contingently, and thus the supposedly synthetic a priori propositions about space, time, and their mathematics would not be necessarily true throughout their domain. This argument thus exploits the key epistemological premise for idealism, namely that any isomorphism between knowledge and the known must be necessary. Kant is arguing that since we have no ground to assert a necessary isomorphism between the spatio-temporal structure of our experience and the spatio-temporal structure in things as they are in themselves, we must deny the latter altogether. Kant makes this key move several times. In the Critique, he poses the rhetorical question,