Seforno: That one sin leads to another.

Metzudas David: What it says at the end of this verse; refer to 7:27:4:10.

Rav Sadya Gaon: The one who gathered, he said

Rashi: The collection is feminine. When Koheles is masculine, it refers to the gatherer, i.e. Shlomo.

Rashi citing Koheles Zuta: Sometimes Ru'ach ha'Kodesh speaks in the masculine, and sometimes in the feminine. It says "Ezri u'Mefaleti", and "Ezrasi u'Mefaleti Atah" (Tehilim 70:6, 40:18).

Ibn Ezra: It refers to his Chachmah.

Rashbam: It refers to Nefesh Koheles. Also "va'Techal David ha'Melech Latzeis" (Shmuel II, 13:39) means va'Techal Nefesh David

Rav Sadya Gaon: Via joining matters one to another, the purpose is found.

Rashi (from Sotah 8b): Tzadikim's Mitzvos and Resha'im's sins, they are counted in front of Hashem one by one, until they join to a great sum.

Ibn Ezra #1: "Cheshbon" is thought. One cannot find what he desires, i.e. thought, until he joins one [idea] to another. From two opinions is born a third. So it is regarding the Nefesh, and all the more so regarding the body. It would be improper for a Chacham to join with a woman at all, if not that this is the only way to leave his form (a child) after him. Our verse discusses a woman, like verse 26.

Ibn Ezra #2: Our verse is tied to the coming verse. Cheshbon is thought, or count. When you think about anything, you do not find it alone, only near something else. E.g. big is only compared to small, and vice-versa. The same applies to Chacham and Kesil, and Tzadik and Rasha. You find a man of intellect, Chachmah and fear - a straight person, only compared to others. You find this only among males, and only one in 1,000.

Rashbam: I joined one idea to another to investigate and find Chachmah of the world.

Ri Kara: This refers to "Sabosi Ani v'Libi Lada'as v'Lasur u'Vakesh Chachmah v'Cheshbon" (25). I wanted to know the calculations of Chachmah, and I found their Chachmah. One who wants to know a great sum, he must begin one by one - a small amount, and he doubles and increases until it reaches a great amount, and he joins it to the calculation that he desires to find.

Rid: I sought Chachmah like one who counts one by one to know the number.

R. Avigdor: Normally, if one is Chayav Misah for a sin, his ox dies, his hen is lost, his flask breaks, his egg breaks, and he stubs his finger and blood exudes. Some blood is like all the blood; some of the Nefesh is like the entire Nefesh. Via joining these afflictions, the calculation [of his liability] is exhausted, and he gets Kaparah.

Seforno: When I sought a considerable number of people to whom this does not occur [that one sin leads to another - refer to 7:28:1:3]