Proverbs 14:7

Three proverbs regarding fools: 7 Go from the presence of a foolish man, And surely thou hast not known lips of knowledge; i.e., surely hast not brought into experience that he possesses lips which express experimental knowledge, or: surely thou must confess on reflection that no prudent word has come forth from his mouth. If 7b were intended to assign a motive, then the expression would be כּי בל־תּדע or וּבל־תּדע (Isa 44:9), according to which Aquila and Theodotion translate, καὶ οὐ μὴ γνῷς. נגד is the sphere of vision, and מנּגד denotes either away from the sphere of vision, as e.g., Isa 1:16, or, inasmuch as מן is used as in מעל, מתּחת, and the like: at a certain distance from the sphere of vision, but so that one keeps the object in sight, Gen 21:16. נגד ל denotes, as the inverted expression Deu 28:66 shows, over against any one, so that he has the object visibly before him, and מנּגד ל, Jdg 20:34, from the neighbourhood of a place where one has it in view. So also here: go away from the vis--vis (vis = visûs) of the foolish man, if thou hast to do with such an one; whence, 7b, follows what he who has gone away must on looking back say to himself. בל (with the pret. as e.g., Isa 33:23) expresses a negative with emphasis. Nolde and others, also Fleischer, interpret 7b relatively: et in quo non cognoveris labia scientiae. If וּבל־ידע were the expression used, then it would be explained after Pro 9:13, for the idea of the foolish man is extended: and of such an one as absolutely knows not how to speak anything prudent. But in וּבל־ידעתּ the relative clause intended must be indicated by the added בּו: and of such an one in whom... Besides, in this case וּלא (vid., Psa 35:15) would have been nearer than וּבל. The lxx has modified this proverb, and yet has brought…

See here, 1. How we may discern a fool and discover him, a wicked man, for he is a foolish man. If we perceive not in him the lips of knowledge, if we find there is no relish or savour of piety in his discourse, that his communication is all corrupt and corrupting, and nothing in it good and to the use of edifying, we may conclude the treasure is bad. 2. How we must decline such a one and depart from him: Go from his presence, for thou perceivest there is no good to be gotten by his company, but danger of getting hurt by it. Sometimes the only way we have of reproving wicked discourse and witnessing against it is by leaving the company and going out of the hearing of it.