This argument has been the subject of a prolonged controversy between Trendelenburg and Kuno Fischer.[463] Trendelenburg was able to prove his main point, namely, that the above argument is quite inconclusive. Kant recognises only two alternatives, either space as objective is known _a posteriori_, or being an _a priori_ representation it is subjective in origin. There exists a third alternative, namely, that though our intuition of space is subjective in origin, space is itself an inherent property of things in themselves. The central thesis of the rationalist philosophy of the Enlightenment was, indeed, that the independently real can be known by _a priori_ thinking. Even granting the validity of Kant’s later conclusion, first drawn in the next paragraph, that space is the subjective form of all external intuition, that would only prove that it does not belong to _appearances_, prior to our apprehension of them; nothing is thereby proved in regard to the character of things in themselves. We anticipate by _a priori_ reasoning only the nature of appearances, never the constitution of things in themselves. Therefore space, even though _a priori_, may belong to the independently real. The above argument cannot prove the given thesis. Vaihinger contends[464] that the reason why Kant does not even attempt to argue in support of the principle, that the _a priori_ must be purely subjective, is that he accepts it as self-evident. This explanation does not, however, seem satisfactory. But Vaihinger supplies the data for modification of his own assertion. It was, it would seem, the existence of the antinomies which first and chiefly led Kant to assert the subjectivity of space and time.[465] For as he then believed that a satisfactory solution of the antinomies is possible only on the assumption of the subjectivity of space and time, he regarded their subjectivity as being conclusively established, and accordingly failed to examine with sufficient care the validity of his additional proof from their apriority. This would seem to be confirmed by the fact that when later,[466] in reply to criticisms of the arguments of the first edition, he so far modified his position as to offer reasons in support of the above general principle, even then he nowhere discussed the principle in reference to the forms of sense. All his discussions concern only the possible independent reality of the forms of thought.[467] To the very last Kant would seem to have regarded the above argument as an independent, and by itself a sufficient, proof of the subjectivity of space.