Conversation so far:
What is Epistemology?
In the field of epistemology, Putnam is known for his "brain in a vat" thought experiment (a modernized version of Descartes's evil demon hypothesis).
What was Putnam's "brain in a vat" experiment?
if some person, Mary, were a "brain in a vat", whose every experience is received through wiring and other gadgetry created by the "mad scientist.
What did the experiment prove?
Putnam has clarified that his real target in this argument was never skepticism, but metaphysical realism.

Question: How did it explain metaphysical realism?
Self-contained rewrite: How did Hilary Putnam's "brain in a vat" experiment in epistemology explain metaphysical realism?