t light. Though
certainly not based on experience, they did not seem to him
self-evident.[21] Hence,[22] in the case of these principles, he
sought to give what he did not seek to give in the case of
mathematical judgements, viz. a proof of their truth.[23] The nerve of
the proof lies in the contention that these principles are involved
not merely in any general judgement in physics, e. g. 'All bodies are
heavy,' but even in any singular judgement, e. g. 'This body is
heavy,' and that the validity of singular judgements is universally
conceded. Thus here the fact upon which he takes his stand is not the
admitted truth of the universal judgements under consideration, but
the admitted truth of any singular judgement in physics. His
treatment, then, of the universal judgements of mathematics and that
of the principles underlying physics are distinguished by the fact
that, while he accepts the former as needing no proof, he seeks to
prove the latter from the admitted validity of singular judgements in
physics. At the same time the acceptance of mathematical judgements
and the proof of the _a priori_ principles of physics have for Kant a
common presupposition which distinguishes mathematics and physics from
metaphysics. Like universal judgements in mathematics, singular
judgements in physics, and therefore the principles which they
presuppose, are true only if the objects to which they relate are
phenomena. Both in mathematics and physics, therefore, it is a
condition of _a priori_ knowledge that it relates to phenomena and not
to things in themselves. But, just for this reason, metaphysics is in
a different position; since God, freedom, and immortality can never be
objects of experience, _a priori_ knowledge in metaphysics, and
therefore metaphysics itself, is impossible. Thus for Kant the very
condition, the realization of which justifies the acceptance of
mathematical judgements and enables us to prove the principles of
physics, involves the impossibility of metaphysics.

[21] This is stated B. 200, M. 121. It is also implied B.
122, M. 75, B. 263-4, M. 160, and by the argument of the
_Analytic_ generally.

[22] This appears to be the real cause of the difference of
treatment, though it is not the reason assigned by Kant
himself, cf. B. 120, M. 73-4.

[23] His remarks about pure natural science in B. 20, M. 13
and Prol. Sec. 4 sub fin., do not represent the normal attitude
of the _