Descartes: A Biography

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The French Liar’s Monkey and the Utrecht Crisis 

body, especially when he suspected that Regius was willing to fudge that

distinction. For example, he wrote to him in June:

Youacknowledge that thought is an attribute of a substance that includes no extension

and, conversely, that extension is an attribute of a substance that includes no thought.

Youmust accept, therefore, that a thinking substance is distinct from an extended

substance. For we have no criterion by which we know that one substance differs from

another except that we understand one without the other. God can patently bring

about anything that we understand clearly....However, we can understand clearly a

thinking substance that is not extended and an extended substance that is not thinking,

as you acknowledge. Even if God joins and unites them as much as he can, he cannot

thereby deprive himself of his omnipotence, nor therefore can he take away his ability

to separate them, and therefore they remain distinct. (iii.)

This real distinction between mind and body was destined to reappear in

subsequent discussions and correspondence. It proved to be a source of

contention between Descartes and Regius in later years. However, long

before Regius went public with his disagreement with Descartes, Princess

Elizabeth was to take up the same question in, and to raise it as

something that she genuinely did not understand.