«Totius artis secretum». The Order of Knowledge and the Order of Being in Descartes’ philosophy

In the "Regulæ", Descartes, refusing traditional categories, defines knowledge as a comparison of terms, which are placed in relation to each other by the intellect according to these same criteria of greater, or lesser easiness and simplicity to be conceived. «The secret of whole art» consists then in finding, in the different series of knowledge, the term that is the most absolute in comparison with the relative terms that are deduced with certainty from the first term only pursuant to the epistemological requirements, in so as to allow the orderly knowledge. No ontological absoluteness, no priority of primary substances, no subordinate relationship of genera and species: only a correlation of terms related to each other in the order and arrangement of the method, this correlation depending upon each man’s judgment ("ex arbitrio") and different inquiries ("quæstiones") the intellect undertakes. In this way, the absolute of a series of knowledge is not the absolute of Being, but the absolute of the concept that can become relative in another series in which the inquiry is different. Nevertheless, while in his theory of knowledge and in his essays of science Descartes is so drastic and radical in rejecting the traditional models of intelligibility based on the genera of Being, he can not escape the proof of that category for election which is the substance in metaphysics. And this happens when Descartes decides to pass, through the principles of metaphysics, from the purely conceptual level of the evidence of the «simple notions» of science to their reality as real beings. In Cartesian philosophy, the status of the substance appears very complex, placed between a significant absence in science, dictated by the raisons of the method, and a significant presence in metaphysics, inspired by a philosophical reflexion on its definition on the ontological level as fundamental entity of reality and on the logic level as subject of predication of attributes and properties. Relatively traditional in its definition, but extremely original in the dualistic univocally Cartesian conception and not less problematic for its different interpretations from the beginning until now, the notion of substance appears relatively late in Descartes’ works: it acquires a real philosophical meaning and operates as an important concept for the first time in the metaphysics of the "Discours" and of the "Meditationes", and it is discussed by Descartes with technical reasons and philosophical lexicon in his letters and in his replays to the authors of the objections to these texts. Nevertheless, an organic and complete Cartesian account of the substance, which loses its name of «category» and acquires the name of "simple notion", is exposed only in the Principia, in that systematic text conceived by Descartes for the learned public of philosophers or students of philosophy.