How Is Descartes' Argument against Scepticism Better than Putnam's?

'If a person can think of an F, then that person has come into causal contact with an F in the right way' is a premise in an obvious reconstruction of Putnam's argument that we are not brains in vats. 'If a person can think of an F, then that person has come into causal contact with an F or with something at least as good as an F' is the only controversial premise in Descartes' argument for the existence of God. Putnam's principle entails Descartes', which suggests that we should enquire after better versions of Putnam's proof. I present three variations and conclude that Putnam's semantic theory does not have anti-sceptical consequences. In contrast, given Descartes' cognitive situation, he was perfectly justified in accepting the soundness of his argument for the existence of God.