Regulative Use of Reason
In the Critique of Pure Reason, Kant introduces, in addition to the transcendental principles of understanding (Transcendental Analytic), the transcendental principles of reasons: these are homogeneity, specification, and continuity. The principles of understanding refer to the “field of possible experience” (A 702/B 730), and in doing so, they are concepts of cognition, whereas the principles of reason lack these references and are therefore treated as concepts of thought. Nevertheless, the principles of reason are not merely logical concepts; they have an independent transcendental function in the process of cognition. In this sense, Kant differentiates between an object-oriented level and a meta or reflexive level, and he develops both with the aid of independent transcendental principles.
Kant’s key suggestion is that ideas enable “entirely new prospects” (A 687/B 715) for the field of experience, which demonstrate the positive influence of the principles and ideas of reason. Among other argumentative strategies, these insights justify the transcendental function in a heuristic-pragmatic way.
Regulative Use of Reason and Metaphysical Foundations of Natural Science
Based on both the principles of understanding and the principles of reason, Kant introduces in the Doctrine of Method (A 832/B 860–A 837/B 865), and particularly in the Preface of the Metaphysical Foundations of Natural Science (MAdN AA IV, 468), a critical philosophy of science (A 832ff./B 860ff.; MAdN AA IV, 468f.). Thus his doctrine of nature includes a pure part, according to which it is established as a so called proper science, and, notwithstanding the above, a whole of cognition, according to which it is a rational science. The principles of understanding guarantee apodictic certainty, and the principles of reason offer the possibility for a critical reflection on the respective disciplines and their status quo, even guided by principles.
In this sense, Kant offers a starting point for understanding rationally justified cognition taking into consideration its ethical and practical conditions, its anthropological and psychological constants, the agreed guidelines of researchers, and the socio-political or economical influences of the society.