In a number of places, Richard Swinburne has defended the logical possibility of perception without a body; and has inferred from this logical possibility that substance dualism is true. I challenge his defence of disembodied perception by arguing that a disembodied perceiver would not be able to distinguish between perceptions and hallucinations. I then claim that even if disembodied perception were possible, this could not be used to support substance dualism: such an inference would be either invalid or question-begging.

I illustrate with three classical examples the mistakes arising from using a modal operator admitting multiple interpretations in the same argument; the flaws arise especially easily if no attention is paid to the range of propositional variables. Premisses taken separately might seem convincing and a substitution for a propositional variable in a modal context might seem legitimate. But there is no single interpretation of the modal operators involved under which all the premisses are plausible and the substitution successful.