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 -

and 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 -

a computer that is feeding you a virtual reality experience

and 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