This article examines certain essential points of Cartesian method and its foundation. (1) If Descartes selects analysis as his method _ and he does it is from the ancient tradition of Greek geometers that we should survey it. (2) However, the nature of his method as such is not mathematical: analysis discloses how reason comes to know, as illustrated by the science of geometry. (3) Cartesian thinking distinguishes methodology and epistemology: relationships of dependency among objects are, in general, established starting from relative and complex ones, given their methodological primacy in relation to the absolute and the simple. (4) One of the most fruitful ways to justify this and, thus, to justify the method of analysis, as far as it is a procedure "against the current" is connected with the Cartesian thesis that affirms that to understand something is to understand its cause, due to the fact that everything that exists, exists as an effect. (5) It is of the essence of the method of analysis, therefore, that it may assume the effect as given and proceed backwards (or upwards) in search of the cause; and its foundation is the axiom of identity between cause and reason, causa sive ratio.
Descartes; Method of analysis; Pappus; Mathematics; Modus operandi of the reason; Causa sive ratio