In philosophy of mathematics, he and his mentor W. V. O. Quine developed the "Quine–Putnam indispensability thesis", an argument for the reality of mathematical entities, later espousing the view that mathematics is not purely logical, but "quasi-empirical". In epistemology, he is known for his critique of the well-known "brain in a vat" thought experiment. This thought experiment appears to provide a powerful argument for epistemological skepticism, but Putnam challenges its coherence. In metaphysics, he originally espoused a position called metaphysical realism, but eventually became one of its most outspoken critics, first adopting a view he called "internal realism", which he later abandoned. Despite these changes of view, throughout his career he remained committed to scientific realism, roughly the view that mature scientific theories are approximately true descriptions of ways things are.