In Meditations II, we see Descartes make the assertion that he must exist whenever he thinks "I think, I am", and the existence of the thinking thing is undoubtable. But as he can still doubt the existence of his body, by Identity of the Indescernibles, the mind and body cannot be the same, as one can be doubted, while the other can't. But Descartes never explicitly invokes this argument for mind body distinction in Meditations? So does he make this anywhere else? Or is it just an implicit argument?