This paper reflects on the status of Descartes' notion of the mind-body union as an object of knowledge in the framework of his new philosophy of nature, and argues that it should be taken seriously as representing a third kind of real thing or reality-that of human nature. Because it does not meet the criteria of distinctness that the two natures composing it-those of thinking minds and extended bodies- meet, the phenomena referred to it, which are objects of psychology as traditionally understood, fall outside the scope of clear and distinct perception required for knowledge. The prospects for rationalist psychology are bleak, since because of the mind-body union so little of the contents of the human mind are accessible to rational inspection or introspection. Mechanistic natural philosophy on the other hand gives us knowledge only of the physiological and corporeal aspects of the phenomena Descartes classifies as mental. What pertains to the mind-body union can only be known through the senses, moreover, we learn to conceive the mind-body union only through daily experience ("usant seulement de la vie et des conversations ordinaries", AT 3, 695). I discuss the nature of this experience, and the sense in which Cartesian psychology without being part of his philosophy of nature in the strict sense of the term, can still he seen as a naturalist undertaking in a more traditional sense of nature where life, sentience, reasoning and rational action are all seen as natural phenomena.
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