When thou perceivest not - the lips of knowledge - Instead of דעת daath, knowledge, several MSS. have שקר sheker, a lie. How this reading came I cannot conjecture. The meaning of the adage is plain: Never associate with a vain, empty fellow, when thou perceivest he can neither convey nor receive instruction.

Go from the presence of a foolish man,.... A wicked one; avoid him, shun his company, depart from him, have no fellowship with him, it, being dangerous, infectious, and hurtful;
when thou perceivest not in him the lips of knowledge; when it is observed that his lips pour out foolishness, what is corrupt and unsavoury, unchaste and filthy; what does not minister grace to the hearers, nor is for the use of edifying, nor any ways improving in useful knowledge, but all the reverse: the Targum is,
"for there is no knowledge in his lips,''
in what is expressed by them; some understand this ironically, and render the words thus, "go right against a foolish man" (f); join in company with him, "and thou shalt not know the lips of knowledge", or learn anything by him; if you have a mind to be ignorant, keep company with a foolish man; so Jarchi and Gersom: or rather to this sense the words may be rendered, "go to a foolish man, seeing thou knowest not the lips of knowledge" (g), since thou dost not approve of wise and knowing men, whose lips would teach knowledge; and despisest the Gospel, and Gospel ministers the pope of Rome, as Cocceius on the text serves, and hear him, what his holiness and infallibility says; or some other false teacher.
(f) "e regione viri stulti", De Dieu; so Gussetius, p. 495. and Schultens (g) "Abi ut stes cora in viro stolido", Cocceius.

We discover a wicked man if there is no savour of piety in his discourse.

Avoid the society of those who cannot teach you.

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 (Isaiah 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., Isaiah 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, Genesis 21:16. נגד ל denotes, as the inverted expression Deuteronomy 28:66 shows, over against any one, so that he has the object visibly before him, and מנּגד ל, Judges 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., Isaiah 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 Proverbs 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., Psalm 35:15) would have been nearer than וּבל. The lxx has modified this proverb, and yet has brought out nothing that is correct; not only the Syr., but also Hitzig follows it, when he translates, "The foolish man hath everything before him, but lips of knowledge are a receptacle of knowledge" (וּכלי דּעת). It racks one's brains to find out the meaning of the first part here, and, as Bttcher rightly says, who can be satisfied with the "lips of knowledge" as the "receptacle of knowledge"?