What is going "mi'Neged l'Ish Kesil"?
Rashi: Do not join to him constantly.
R. Yonah: Distance from one who is known to be foolish - the actions of Evvil and bad Midos.
Malbim: A Letz (verse 6) does not mock Da'as. He mocks the laws of Chachmah, for there is no proof for them. They are received, without clear Da'as. If he would understand with clear Da'as, he would not scoff! Because he sought Chachmah, but it is not [in him], he does not come to Tevunah, which is after Chachmah. If he had Tevunah, it would be easy for him to reach Da'as! A Kesil is different. He received the laws of Chachmah, just desire overpowered him. He hates the laws of Chachmah, for they oppose his desire - "u'Chesilim Yisne'u Da'as", "Ta'avah Nihyah Te'erav l'Nafesh v'So'avas Kesilim Sur me'Ra" (1:22, 13:19). He schemes to put doubts in the laws of Chachmah, to have a pretext for going in the freedom of his heart. Refer to 14:7:2:3.
What is the meaning of "u'Val Yadata"?
Rashi: In the end, you will not know Chachmah.
R. Yonah: The prefix Lamed in "l'Ish" applies also to this. The pronoun is omitted, like "v'Lakachta Lach 1 " (Devarim 7:25, 21:11). Distance from one that you do not know that he has lips of Da'as. Friendship with him is dangerous. You must be concerned for the doubt, and not join to him until you investigate whether he is careful not to reveal secrets, and he does not revile and disgrace, amidst anger or pride.
Malbim: Regarding a Letz - you, who know Sifsei Da'as, can go with him. He does not mock Da'as, only Chachmah. However, do not go with a Kesil, who does not accept also Da'as, lest the Da'as that you acquired until now, you will be estranged from it; it will seem that you do not know it at all. Joining to him will lead you to learn to follow desire and what is pleasant - "v'Ro'eh Kesilim Yero'a" (13:20).
Magihah: There it should say v'Lakachta Osah Lach; here it should say u'Val Yadata Oso.
