Freedom has a fundamental systematic value in Kant s philosophy. In practical philosophy, it is a condition of action in an eminent sense. In the Groundwork of the Metaphysics of Morals and the Critique of Practical Reason, Kant tries to prove freedom. The argumentation in both books is different. The present work focuses mainly on the justification of the second Critique. In it, Kant states that we know about freedom thanks to the fact of moral conscience or a law that binds the subject unconditionally. An attempt will be made to show, from a consideration of Kant s texts, the works of some interpreters, as well as a systematic reflection, that freedom, and the fact of moral conscience, require access to practical spontaneity on the part of of the subject, an issue that Kant leaves suggested in some passages.