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Published on Oct 7, 2012
Examining the view that mind and body are separate substances.
Note at 7:08 A reductio ad absurdum argument (one which attributes a machine with thought purely for the sake of argument, to demonstrate that genuinely absurd / contradictory consequences follow) would be valid. We can see immediately that Plantinga's thought experiment doesn't achieve this: failure to discern how a thinking machine is thinking indicates only lack of comprehension, not a genuine absurdity / contradiction.
But his use of Leibniz' scenario isn't valid. Leibniz doesn't just propose a thinking machine, but one we can enter and inspect. If physical thinking things are impossible - as Plantinga claims - then whatever machine we conjure up in our imagination to enter and inspect, it can't be a genuine physical thinking thing, just as it would be impossible to inspect a machine that prints square circles. (Besides, if there's truly nothing we could be faced with inside the machine that would signal thought, it makes no sense to ask us to inspect it, since no inspection could help us discern thinking machines from non-thinking ones anyway.) It is this sense in which Plantinga cannot use thinking machines to show machines can't think. His argument is incoherent. It is certainly not a valid reductio ad absurdum.