Many propositions are not known to be true or false, and many phenomena are not understood. What determines what propositions and phenomena are perceived as knowable or unknowable? We tested whether factors related to scientific methodology (a proposition’s reducibility and falsifiability), its intrinsic metaphysics (the materiality of the phenomena and its scope of applicability), and its relation to other knowledge (its centrality to one’s other beliefs and values) influence knowability. Across a wide range of naturalistic scientific and pseudoscientific phenomena (Studies 1 and 2), as well as artificial stimuli (Study 3), we found that reducibility and falsifiability have strong direct effects on knowability, that materiality and scope have strong indirect effects (via reducibility and falsifiability), and that belief and value centrality have inconsistent and weak effects on knowability. We conclude that people evaluate the knowability of propositions consistently with principles proposed by epistemologists and practicing scientists.