μὴ γὰρ οὐ φαῦλον ᾖ ὃ λέγει
For there might well be a great deal in what he says.
The present subjunctive in the main clause, preceded by μή, signals a cautiously expressed assertion, often with an ironic nuance.
In translation perhaps or presumably may be used. It is also possible to express the elided clause of fearing: ‘I fear that…’ (or ‘I suspect that…’).
The negation is (μὴ) οὐ.
Beside the conjunctive the (present or aorist) indicative also occurs with this meaning, albeit less frequently and only with a negated μὴ οὐ.
This construction is current from the 5th century B.C. onwards and is regarded as typically Platonic. Historically it may be explained as a case of insubordination, i.e. in which an (obligatory) subordinate clause becomes a main clause through ellipsis of the main verb: [φοβοῦμαι] μὴ (οὐ) φαῦλον ἦ ‘[I fear] that it is not stupid.’
The presence of the indicative in later instances proves that Greek speakers no longer regarded this construction as a case of insubordination.