This is a re-post from 2014.
Descartes gives the principle 'God is not a deceiver' an important role in his argument for the existence of bodies. Thomas Hobbes in the Third Objections replied:
The standard view is that doctors are not at fault if they deceive their patients for their health's sake, and that fathers are not at fault if they deceive their children for their own good. For the crime of deception consists not in the falsity of what is said but in the harm done by the deceiver. M. Descartes should thus consider the proposition 'God can in no case deceive us' and see whether it is universally true. For if it is not universally true, the conclusion 'Therefore corporeal things exist' does not follow. (AT VII, 195; CSM II, 136)
Descartes dismisses this:
My conclusion does not require that we can in no case be deceived (indeed, I have readily admitted that we are often deceived). All that it requires is that we are not deceived in cases where our going wrong would suggest an intention to deceive on the part of God; for it is self-contradictory that God should have such an intention. Once more my opponent's reasoning is invalid. (AT VII, 195; CSM II, 136-137)
One reason this exchange is interesting is that we seem to see the influence of the difference between Protestant and Catholic moral philosophy and theology here. Hobbes in talking about the "standard view" is in fact talking about what was the standard view in Protestant jurisprudence, as we find it in people like Grotius, who formulates the wrongness of deception in terms of a right to truth that one may or may not have depending on the circumstances. Descartes, on the other hand, is taking a more Catholic approach, in which defective intention is the central idea in discussions of deception.
