Abstract: The path of Kant's reconstruction of metaphysics is to distinguish the field of knowledge from the field of practice through demarcation.In this way,metaphysical concepts such as God and soul,which are denied in the field of knowledge,are rebuilt in the field of morality.Therefore,theoretical rationality is inevitable,while practical rationality proves freedom.However,the two rationality are not unrelated,but the performance of the same rationality in different aspects.The rationalism foundation can be found in both epistemology and ethics,so Kant put forward the proposition that "the natural world is the model of the rational world" and "the moral law should be like the natural law".Kant's three critical systems are harmonious and coherent,and jointly construct a complete framework of rational edifice.

Key words: freedom, inevitability, rationality, morality, knowledge, Kant