Who is a "Pesi"?
R. Yonah: Pesi is one who did not yet learn Chachmah - "Pesayos u'Val Yad'ah Mah" (9:13), "Pesi Ya'amin lChol Davar" (14:15). We find that "Al Yochach Letz Pen Yisna'echa" (9:8), "b'Aznei Chesil Al Tedaber" (23:9). Just like it is a Mitzvah to say something that will be heeded, is a Mitzvah not to say something that will not be heeded (Yevamos 65b). Therefore, do not rebuke a fool. Since his deeds are Aveiros, and he accustomed himself to pursue his desire, it became his nature - "Gam Atem Tuchlu Leheitiv Limudei Hare'a" (Yirmeyah 13:23, i.e. you cannot change). However, one who sins occasionally, when the Yetzer overpowers him, even though he is Mezid, it is proper to rebuke him. Since he did not accustom himself to this sin, perhaps he will hear rebuke. He is a Pesi, for he is Misfateh (accepts enticement of) his Yetzer.
What will give Armah to fools?
Rashi: These Meshalim.
R. Yonah: First one must know Musar, to take it and fulfill it in practice. Afterwards he can rebuke the fools - first correct yourself before correcting others (Bava Metzi'a 107b)!
Malbim: Armah is the opposite of being a Pesi, who believes everything. An Arum understands to verify. With Armah, his heart is not enticed to everything; he tests everything with his Sechel. Armah is always opposite Pesi. These Meshalim will develop Armah in a Pesi's heart. He will examine his ways, and not be like a silly dove.
Who is a "Na'ar"?
Rashi: It is one who is Menu'ar (empty) of all learning; he did not yet learn anything.
R. Yonah: It is a youth. One is commanded to rebuke him, even though he accustomed himself to sin for many days, for he is under his father's hand to chastise him and hit him. Also, there is hope that he will rule over his desire and change his nature, since he is still a youth - "Iveles Keshurah v'Lev Na'ar Shevet Musar Yarchikenah Mimenu" (22:15). He is not a Pesi; he has Armah, but since he is young, he lacks Da'as and Mezimah (refer to 1:4:4:2). Da'as is acquired via much testing with senses or known concepts; a youth did not do this enough to acquire a clear Yedi'ah.
What is "Mezimah"?
Rashi: It is thought of a counsel.
R. Yonah: These are deep thoughts that are not yet found in a Na'ar's heart. These Meshalim will give to him both Da'as to test every matter and know it, and also Mezimah.
