The letter of February 21, 1772 by Immanuel Kant to his friend and student Marcus Herz is recognized by all scholars of Kantian philosophy as one of the most important documents we have regarding the development of his thought. With the Dissertation of 1770, Kant had already laid the foundations of future Transcendental Aesthetic, and with this letter he raises the fundamental problems of Analytic of concepts. There are two topics that occupy him during the period of writing the letter: the formation of the table of categories, or otherwise the metaphysical production of pure concepts of understanding, and the justification of the objectivity of pure concepts of understanding, or otherwise their transcendental production. The thorny problem that Kant faces in 1772 is the relationship between pure concepts of understanding and reality, and by reality we mean things in themselves. The issue therefore is to learn how there can be agreement between an idea and a thing, without them being in a cause/effect relationship. In the Kantian corpus, the letter of February 21, 1772 marks the end of the pre-critical period and the beginning of critical philosophy, which is why we deemed it appropriate to accompany the edition with a study by the distinguished historian of Kantian philosophy, Herman J. De Vleeschauwer, where all the parameters and the stakes of this letter are analyzed in depth.