Torah Temimah citing Yuma 19b: One who speaks secular matters (R. Avraham Min ha'Har - without need) transgresses a Lav "Lo Yuchal Ledaber." (This is an Asmachta.)

Rav Sadya Gaon: All worldly matters. Man's speech cannot encompass them.

Rashi: This refers to "Mah Yisron?" (3). If man switches [engaging in] Torah for idle matters, they are wearisome ¹ , and he cannot attain all of them.

Ibn Ezra: The four 'elements', even if they move, they return like they were. Therefore, what is composed from them, will not persist in its composition. Therefore, one cannot tell them.

Ri Kara: Many things are never Batel. If I would come to tell them, they would weary me, for they are uncountable - "v'Lahag Harbe Yegi'as Basar" (12:12).

Rid: Whatever one engages in in this world, it wearies and distracts him to the point that he cannot speak.

Seforno: All matters that weary, for their strength is limited - unlike Heavenly matters, which do not weary - they are too many to be told. Perpetuity of the species is not due to their nature, which wears out. Rather, an Overseer who does not weary, He ensures their continuity.

Metzudas David: When man attains his hope, he rests and ceases to toil. When he does not attain, he toils without limit. All worldly matters weary people, for no one attains his hope. If I wanted to list all such matters, I could not. They are too many for one to tell all of them.

Rav Sadya Gaon: The eye is not satiated with seeing worldly matters, and the ear is not full (tired) from hearing them.

Rashi: If one comes to engage in worldly matters, the eye will not be satiated, and the ear will not full [from seeing and hearing them].

Ibn Ezra: The eye sees pictures in clear air that are only for a moment. Also sounds that one hears do not persist. Therefore, man cannot know all the details; only the Creator can know them.

Rashbam: The eye is not satiated, for it always looks towards the future. The ear always wants to hear deeds done in the world. It discusses the ear being filled, for it is hollow.

Ri Kara: The eye is not satiated, for whatever it sees looks new. The ear is not full, for whatever he hears, it seems to him that no one ever heard like this. This is not so - "Mah she'Hayah Hu she'Yihyeh" (9).

Rid: There is no loss via not toiling for worldly matters. Even if he had all the gold and silver in the world, his eye would not be satiated; he would want to toil and acquire more. If he would rule over all kingdoms in the world, his ear would not be full; he would want to toil and conquer more, if he would find [what to conquer]. What is it worth - in the end, "Dor Holech v'Dor Ba" (4 - he will die)!

Seforno: The eye of His Hashgachah is not satiated or wearied from seeing and supervising, and nature does not cease to hear His commands.

Metzudas David: This is a Mashal. If a man is very satiated, he cannot eat more. If one would see these matters (in which one does not attain his hope), the eye will not be satiated to the point that it cannot see more, for they all finished. Really, there is more to see. Also a full ear is a Mashal of a full Kli that one cannot put more in it. The ear will never be full of hearing, that there is nothing more to hear; really, there is more to hear.