When Descartes made his concession to Elisabeth—telling her, in effect, that it was all right to regard the soul as material—he immediately added that “one would not cease to know…that the soul is separable from [the body].” This suggests that what mattered most to him in his “dualism” was providing support for immortality. Many of the details were, perhaps, negotiable. Further evidence of the role that immortality played in his thinking about the mind is found in some remarks he made about animals. Writing to William Cavendish, the first marquess of Newcastle, who was particularly fond of horses, Descartes confessed that he found it hard to believe that beasts can have mental lives like ours, because “if they thought as we do, they would have an immortal soul like us.” This is unlikely, Descartes continued, because if you believe this of some creatures, there is no reason not to believe it of all, and “many of them, such as oysters and sponges, are too imperfect for that to be credible.”
What was to have been Descartes’ final word on the relation between mind and body never appeared, quite possibly because he was struggling to address Elisabeth’s difficulties. When Principles of Philosophy was published in 1644, it contained only four parts of a projected six. The first four dealt with “The Principles of Human Knowledge,” “The Principles of Material Things,” “The Visible Universe,” and “The Earth.” Parts five and six were to have covered “Plants and Animals” and “Man,” but Descartes wrote, “I am not yet completely clear about all the matters I would like to deal with there.”
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