Through the thought experiment of "brain in a vat" and "twin earth" to refute Descartes' skepticism, and even investigate the dispute between realism and nominalism. The former regards the universality of things ( Universal) as a Platonic entity, which the latter denies its real existence. The presupposed transcendental perspective makes Putnam's "referential theory" self-paradoxical, and also makes this reductive absurdity argument based on Semantic Externalism appear to be split and self-defeating.