Pyrrhonism: The doctrine that all knowledge is uncertain, that you can't trust anything you think you know. More commonly used today simply to indicate extreme skepticism. Pyrrhonism comes from the doctrines of Pyrrho the Skeptic, a Greek thinker from the third and fourth century BC.
If you need an adjective, use Pyrrhonic, not (as I mistakenly have done) Pyrrhic. A Pyrrhic victory is a victory that comes at too great a cost, not a victory whose winners can be doubted. Pyrrhic (as a lowercase common noun) is also a measure of meter in writing indicating a metrical foot made up of two short or unaccented syllables.