 One historically predominant metaphysical interest has to do with identifying the nature and the constitution of the soul. Partly for practical reasons, partly for theoretical explanation, reason forms the idea of a metaphysically simple being, the soul. Such an idea is motivated by reason’s demand for the unconditioned. Kant puts this point in a number of ways, suggesting that the idea of the soul is one to which we are led necessarily insofar as we are constrained by reason to seek the “totality” of the “synthesis of conditions of a thought in general” (A397), or insofar as we seek to represent “the unconditioned unity” of “subjective conditions of representations in general” (A406/B433). More straightforwardly, Kant states that a metaphysics of the soul is generated by the demand for the “absolute (unconditioned) unity of the thinking subject itself” (A334/B391). The branch of metaphysics devoted to this topic is Rational Psychology. Rational psychologists, among whom Descartes or Leibniz would serve as apt historical examples, seek to demonstrate, for example, the substantiality, simplicity, and personal identity of the soul. Each such inference, however, involves concluding “from the transcendental concept of the subject, which contains nothing manifold, the absolute unity of this subject itself, of which I possess no concept whatsoever” (A340/B398). In other words, Kant takes the rational psychologist to slide (mistakenly) from formal features of our self concept to material or substantive metaphysical claims about an alleged (super-sensible) object (the soul).