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Ἓν οἶδα, ὅτι οὐδὲν οἶδα → I know only one thing, that I know nothing | all I know is that I know nothing.
off-hand: P. and V. φαύλως, P. ἐξ ἐπιδρομῆς, ἐξ ὑπογυίου.