  First, Kant suggests that in special metaphysics the principles of the transcendental part “are applied to the two species of objects of our senses” (4:470). Thus, the particular kinds of things that could be investigated in a special metaphysics are (i) the objects of outer sense, i.e., matter, and (ii) the objects of inner sense, i.e., thinking beings, which would thus result in a doctrine of body and a doctrine of soul. Kant then argues that because “the possibility of determinate natural things cannot be cognized from their mere concepts … it is still required that the intuition corresponding to the concept be given a priori, that is, that the concept be constructed” (4:470), which is a task that requires mathematics. This is Kant's justification for his famous claim that “in any special doctrine of nature there can be only as much proper science as there is mathematics therein” (4:470). This argument suggests that the necessity required of the pure part of natural science derives from the necessity of the rules by which the mathematical construction of determinate things must proceed.