Why does it say "Arum Koseh Da'as"?
Rashi: He is modest even in his Chachmah. All the more so, he hides his matters of folly!
R. Yonah: This discusses the Midah of modesty. A Chacham hides also his Tzedakos and good deeds, even though they are glory for those who do them - "v'Hatzne'a Leches Im Elokecha" (Michah 6:8).
Malbim: The Arum is not a Chacham. He did not receive the details of the laws of Chachmah, but via prudence he does not do what there is a doubt about it. He gauges his way constantly, and avoids any path with doubt - "Chachmas Arum Havin Darko v'Iveles Kesilim Mirmah" (14:8). The Iveles and doubts that Kesilim make, it is a ruse. They deceive themselves. They know that their folly has no substance. An Arum sees the future and does not go in a doubtful path in darkness, until he understands his way. "Nachalu Pesayim Iveles va'Arumim Yachtiru Da'as", "Kol Arum Ya'aseh v'Da'as u'Chsil Yifrosh Iveles" (14:18, 13:16). Here it explains another difference between Arum and Chacham. The Arum covers Da'as. Also something that he knows, he hides his knowledge from everyone. "Chachamim Yitzpenu Da'as" (10:14); they do not totally cover it. They know Da'as via the laws of Chachmah that they receive, and teach them to the Rabim. The Arum knows via his investigation and guards it for himself, but not for others.
Above, it says "Chachamim Yitzpenu Da'as" (10:14).Why does it say here "Arum Koseh Da'as"?
Malbim (10:14): Tzofem does not totally cover the matter, just he puts it in a place where people cannot see it, but it is exposed. Lechasos (to cover) Da'as is the action of Armah, and not of Chachmah. They do not hide it from proper people - "Leshon Chachamim Teitiv Da'as", "Sifsei Chachamim Yezaru Da'as" (15:2, 7). Tzefinah includes guarding Da'as from improper people.
Why is Lev Kesilim called Iveles?
Rashi: It announces "Iveles" (folly) in a loud voice.
R. Yonah: A Kesil does not hide also his folly, even though it is his detriment. Rather, he announces and informs it to all. It discusses their heart, and not their mouth, for once folly crosses his heart, he announces it immediately.
Malbim: The Evvil is unsure about everything, including the laws of Chachmah. He does not believe in them because he does not understand them. A Kesil knows the laws of Chachmah, just his desire overpowers him, so he does not follow the laws of Chachmah - "Ta'avah Nihyah Te'erav l'Nafesh v'So'avas Kesilim Sur me'Ra" (13:19). Even though he knows that it is evil, it is an abomination for him to veer from evil, for his desire urges him. Therefore he seeks excuses and puts doubts in the laws of Chachmah. Da'as is clear knowledge, the opposite of Iveles (doubting everything). The Kesil does not want Da'as. He prefers doubts, to cover up for doing his heart's desire - "Lech mi'Neged l'Ish Kesil u'Val Yadata Sifsei Da'as" (14:7). "Leshon Chachamim Teitiv Da'as u'Fi Kesilim Yabi'a Iveles", "Lev Navon Yevakesh Da'as u'Fi Kesilim Yir'eh Iveles" (15:2, 14). His heart, the power that rules in him, urges him to desire, and he fears punishment. Therefore, he calls to Iveles to doubt Hashem's punishment and the laws of Chachmah with which He conducts His world. Via this, he is confident - "Bote'ach b'Libo Hu Kesil" (28:26). Also, it is a cover (excuse) for his folly.
