Jamieson, Fausset and Brown on Ecclesiastes 3:19

19. Literally, "For the sons of men (Adam) are a mere chance, as also the beast is a mere chance." These words can only be the sentiments of the skeptical oppressors. God's delay in judgment gives scope for the "manifestation" of their infidelity (Ec 8:11; Psa 55:19; 2Pe 3:3,4). They are "brute beasts," morally (Ec 3:18; Jude 10); and they end by maintaining that man, physically, has no pre-eminence over the beast, both alike being "fortuities." Probably this was the language of Solomon himself in his apostasy. He answers it in Ec 3:21. If Ec 3:19, 20 be his words, they express only that as regards liability to death, excluding the future judgment, as the skeptic oppressors do, man is on a level with the beast. Life is "vanity," if regarded independently of religion. But Ec 3:21 points out the vast difference between them in respect to the future destiny; also (Ec 3:17) beasts have no "judgment" to come.

breath--vitality.