The "brain-in-a-vat hypothesis" has been used by
contemporary philosophers such as Jonathon Dancy
to argue for the notion that only one thing that can
be known and verified.

That one thing is the existence of the self.



The brain-in-a-vat hypothesis goes like this -

there is no way to disprove the possibility that
you are a disembodied brain, suspended in a
solution, in a vat, and wired to a computer
that is feeding you a virtual reality experience.

Further, since you can not rule out the possibility
that you are a brain in a vat, you must be skeptical
of all that you believe to be true -

The ONLY thing you can know for sure is that
you exist, as a consciousness, a Self.