The term persona appears only once in the original Latin text of the Meditations, in the Fourth Meditation (AT VII 61, CSM II 43). There Descartes employs the term to mean “one's role in life,” deriving from persona in the original theatrical sense of “a mask,” as in the early Private Thoughts (AT X 213, CSMK I 2). Nowhere in the Meditations and Objections and Replies, or elsewhere in the published writings, does Descartes explain his conception of personhood or address the problem of “personal identity” that has exercised philosophers since Locke. It is all the more important therefore to determine what he took a “person” to be.
One suggestion that many commentators find appealing is that a Cartesian person is a mind or soul and thus that personal identity consists in sameness of soul (see, e.g., Thiel 2011, 270). In the Second Replies, on the question of whether God could be a deceiver, Descartes notes that in the Meditations he “had a special obligation” to talk about God in a philosophical way, “since there my supposition was that no other human beings were yet known to me, and moreover I was considering myself not as consisting of mind and body but solely as a mind” (AT VII 142–43, CSM II 102). The Meditations are primarily those of a res cogitans, not of a Cartesian human being, whose nature emerges in a letter to Mesland (February 9, 1645). When we talk about a human body, we do not mean just a determinate part of matter with a determinate size, but that part as united to the soul (l’âme). We believe that we have had the same body since infancy, despite its lifelong changes in quantity, shape, and composition. Our body is the same numerically (idem numero) because it is informed [informé] by the same soul (AT IV 166–67, CSMK 243). So is a Cartesian person a human being instead?
A human being, qua substantial union of a body and a mind, is still not a rounded person, because persons have moral and social commitments. Even in the Passions of the Soul (1649), Descartes explains that his purpose is “not to explain the passions as an orator, nor even as a moral philosopher, but solely as a physicist [physicien]” (AT XI 326, CSM I 7).