Hidden from all but a small circle of initiates. The idea that Plato, Aristotle, or almost any great philosopher was concealing the ‘true’ doctrine from all except initiates is well-received in a world which likes codes, conspiracies, and cryptograms; it flatters would-be insiders, and acts as a general excuse to interpret writers as meaning the opposite of what a natural reading of their works would suggest. See also Strauss, Leo.