The subject matter of the article is the concept of "the real use of reason" (usus realis) 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

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... s production of ideas and principles) proves itself as an unavoidable condition for the regulative use of ideas as well as a condition for the production of a critical metaphysics.