[scifiducky]

This critique misses the point of Descartes. It can be reformulated as something like "a thought has happened, therefore we can know at least that something that thinks exists." Getting caught up in the subject-object semantics has no bearing on Descartes approach to objectivity. This is no more tautological than seeing a car and then concluding that cars exist.

[viccis]

How do you define "exists"?

Remember, this is about Cartesian duality (mind-brain duality), so the key question here is not whether a brain exists, but whether the mind exists independently of it.