The subject matter of the article is the concept of “the real use of reason” alluded to by Kant in Critique of Pure Reason A299/b355 and in A305/b362. After comparing it with the “real use of understanding” examined in De mundi sensibilis and in the Critique of Pure Reason, the real use of reason is presented as a legitimate and useful performance that should be distinguished from the deceiving illusion induced by an appearance generated by reason itself. The real use (...) of reason proves itself as an unavoidable condition for the regulative use of ideas as well as a condition for the production of a critical metaphysics. (shrink)