How should he express trust in Hashem?
Rashi: Spend your money to seek a Rav to teach you, "and do not rely on your understanding."
R. Yonah: Trust in Him with a full heart, in truth. "Da Es Elokei Avicha v'Avdehu b'Lev Shalem uv'Nefesh Chafetzah" (Divrei ha'Yamim I, 28:9). B'Lev Shalem and b'Chol Libecha means with a lasting Emunah, without doubts.
Malbim: He must go according to the Mitzvos and Chukim that He commanded, and His Chachmah, and not rely on his own Binah at all.
Why does it add "v'El Binasecha Al Tisha'en"?
Rashi: Refer to 3:5:1:1.
R. Yonah: This is full trust. He trusts in Hashem, and not in man, and also not in his own strength and Sechel. Many err to rely on their understanding. Therefore, the Sefer warns about this more than about relying on wealth and strength. Also, Binah includes wealth and strength, for one relies on them only if his Binah says that one can trust in them. If it seems to you that your act will succeed based on your counsel and strategy, do not rely on this. It all depends on Shamayim - "Meshiv Chachamim Achor v'Daitam Yesakel" (Yeshayah 44:25), "Rabos Machashavos b'Lev Ish va'Atzas Hashem Hi Sakum" (below, 19:21), "Im Hashem Lo Yivneh Vayis Shav Amlu Vonav" (Tehilim 127:1). All our deeds are folly; "Ki Lo la'Kalim ha'Merotz" (Koheles 9:11), "l'Adam Ma'archei Lev ume'Hashem Ma'anei Lashon" (below, 16:1). Even eloquent speech is not in man's hand, and all the more so action!
Malbim: The laws of Chachmah are above human understanding. He cannot attain them with his power of Binah. If he will rely on his Binah, it will lead him to the opposite of the ways of Chachmah. Man attains Chachmah only from Hashem.
Lev Eliyahu (Shemos p.64, citing the Vilna Gaon): Do not say, I will rely on Hashem as much as I am obligated, and also rely on my Sechel. Rather, totally rely on Hashem.
