The transcendental ideal

Why is the system as the philosopher''s foundational discourse split into a doctrinal and a critical system that is into a body of knowledge and a critique of that knowledge? Transcendental reflection in the Kantian sense wants to link a given body of knowledge and the criticism of it; it is metaphysics of morals and metaphysics of nature.In contrast to Fichte who in the Doctrine of Science (1794) wanted to bring together the three Critiques into an absolute self in order to go beyond the categories and the moral law Kant claims that if for doctrinal discourse systematicity is based on the categories of pure intuition i.e. on a metaphysics of nature then conversely critical discourse is the basis of doctrinal discourse and consequently there cannot be doctrinal assertive propositions that are based on the self.