 Proponents of closure might accept both K and GK, perhaps further qualified in natural ways (but they might not: see the concerns about justification closure raised in section 6). By contrast, Fred Dretske and Robert Nozick reject K and therefore GK as well. They reject any closure principle, no matter how narrowly restricted, that warrants our knowing that skeptical hypotheses (e.g., I am a brain in a vat) are false on the basis of mundane knowledge claims (e.g., I am not in a vat). In addition to rejecting K and GK, they deny knowledge closure across instantiation and simplification, but not across equivalence (Nozick 1981: 227–229): 