In this article the kantian Rechtslehre is interpreted as a critical juridical doctrine, understandable under the critical project – started in Kritik der reinen Vernunft and adapted to the practical field in Kritik der praktischen Vernunft. In particular, it is aimed to highlight, besides the apriority, the systematic character and the search for the completeness of the juridical principles, the use of the solubility of problems theory of the reasoning in general in the prolegomenon of Rechtslehre. The study of this preliminary part is justified in presenting the supreme division of the system according to principals, where it is derived a division of the law doctrine, which determines the object (Gegenstand) and, therefore, the field of this particular science and the discussion of the research procedure. Such a priori frame of the Law doctrine is the basis of the following Kantian theory of private law and public law. In a bigger picture, this article can be understood as a renouncement to the idea that the Kantian Rechtslehre does not follow the requirements of the critical philosophy – created by Hermann Cohen (Ethik des reinen Willens, 1904) and detailed by Christian Ritter (Der Rechtsgedanke Kants nach den frühen Quellen, 1971).