Useful commentary:
Ecclesiates 3 (18-21) The common fate of animals and humans under the sun.
I said in my heart, “Concerning the condition of the sons of men, God tests them, that they may see that they themselves are like animals.” For what happens to the sons of men also happens to animals; one thing befalls them: as one dies, so dies the other. Surely, they all have one breath; man has no advantage over animals, for all is vanity. All go to one place: all are from the dust, and all return to dust. Who knows the spirit of the sons of men, which goes upward, and the spirit of the animal, which goes down to the earth?
a.
Concerning the condition of the sons of men… they themselves are like animals: Solomon looked at life among both humans and animals, and also compared their deaths – doing so in
under the sun, absent eternity terms. On this basis, he could say that there is little difference in the life and destiny between humans and animals.
i.
They themselves are like animals: “The pronoun is repeated emphatically, ‘that they themselves are [like] beasts, they in themselves.’” (Deane)
ii. “In their context these verses say that God makes all sensible people realize that they are as much subject to death as is the animal world.” (Wright)
b.
As one dies, so dies the other: The Preacher thought of an animal dying and its body decomposing. Then he thought that by all outward appearance, the same happens to a human body. Therefore,
they all have one breath; man has no advantage over animals, for all is vanity.
i. “The distinction between man and beast in annulled by death; the former’s boasted superiority, his power of conceiving and planning, his greatness, skill, strength, cunning, all come under the category of vanity, as they cannot ward off the inevitable blow.” (Deane)
ii. This is no argument for the doctrine of annihilationism, the idea that the unrighteous dead simply cease to exist, either immediately or after some time of punishment. It is no argument for two reasons. First, Solomon spoke here as a man unconvinced of eternity and the meaning it brings to life. Second, we believe what
2 Timothy 1:10 says: that Jesus
brought life and immortality to light through the gospel. The understanding of the afterlife is cloudy and uncertain in the Old Testament, but much clearer in the New Testament.
c.
Who knows the spirit of the sons of men, which goes upward, and the spirit of the animal, which goes down to the earth: We sense that the Preacher
hoped there was a different destiny between people and animals. Yet in his
under the sun thinking, there was no real reason to believe it – so, “
Who knows”?
i. “What is meant by ‘upward’ and ‘downward’ may be seen by reference to the gnome in
Proverbs 15:24, ‘To the wise the way of life goeth upward, that he may depart from Sheol beneath.’” (Deane)
ii. “The Teacher is speaking phenomenologically, i.e., as things appear to the senses.” (Wright)
https://enduringword.com/bible-commentary/ecclesiastes-3/