From epistemology to dissections: Descartes' two-step argument for a distinction between primary and secondary qualities

Mattia Mantovani

Humboldt University, Berlin

In his seminal Il Saggiatore (1623), Galileo famously claimed that, contrary to triangles and circles, which are the real characters in which the book of Nature is written, flavours and colours do not exist independently of the perceiving subject. The possibility to distinguish between two classes of ideas of the material objects was indeed one of the most debated issues of the Early Modern Age. While fiercely rejected by many, Descartes, on his part, heralded it enthusiastically, by devising a system in philosophy which could account for the distinction and, hence, for his geometrical physics. The aim of my talk is to tease out Descartes' argumentative strategy on the matter, by telling apart the purely epistemological step of his reasoning from the empirical one, that is to say, from the physiological experiments performed by Descartes in order to prove his metaphysical theory about material bodies. My first point is that Descartes' list of the real attributes of a body coincides as a matter of fact with Aristotle's list of the common sensibles or, more precisely, with the more succinct formulation of the list advocated by the Scholastic tradition. Although many interpreters have not taken notice of it, Descartes explicitly subscribes to the Aristotelian distinction between proper and common sensibles and openly states that the latter (magnitude, shape, number, motion and rest) correspond to the only properties of a body admitted by his philosophy. I claim that this theory about the sensibles plays a pivotal role in Descartes' philosophy, even though he departs from the strict Aristotelian concept of a sensible common to all specific senses, by turning this particular class of objects into the “objectum puræ matheseos”. I expound the theory about mathematical knowledge that enables Descartes to conclude that magnitude, shape and the like (the so-called primary ideas) do in fact properly correspond to the real attributes of the material objects, which are hence to be conceived as res extensæ. As my second point, I show that Descartes explicitly maintains that there are no additional purely epistemological arguments which could account for the actual nature of the secondary ideas. Therefore, Descartes has to resort to an alternative line of reasoning and, more precisely, to an empirical one. This argument starts from the assumption that the material world is the subject matter for a geometrical physics (which has just been proved) in order to conclude that the world is nothing but geometrical – which is to say, that colours and all proper sensibles are not real attributes of the material objects. Indeed, to advance this claim Descartes has to take advantage of his whole system of natural philosophy and, more in particular, of his physiological experiments. Finally, I briefly contrast Descartes' two-step argument for a distinction between primary and secondary ideas with Locke's purely empirical one, to show how these two different foundational strategies result in two radically alternative lists of the primary qualities of a body.