Shared ground
The speaker reflects privately (“I said in my heart”) that God “tests” human beings in a way that forces a humbling realization: regarding death, humans resemble animals (v.18–19). That claim is argued from everyday observation, not from abstract theory: both humans and animals die, both share “one breath,” and both end in the same place—dust (v.19–20). The refrain “all is vanity” reinforces Ecclesiastes’ theme that death undercuts the idea of lasting “advantage” from human status or achievement (v.19).
The passage ends by drawing a boundary around what can be known with certainty. It asks who really knows the “spirit”’s direction after death for humans and animals (v.21). Explicitly, the text is not offering a detailed map of the afterlife; it is highlighting limits on confident human claims.
Where interpretation differs (only where needed)
1) What “tests” means (v.18). Some read “tests” as God actively arranging circumstances to expose human limits and puncture pride. Others take it more as God allowing life’s realities to demonstrate the point—less like an exam God gives and more like the world as a mirror.
2) What “one breath” means (v.19). Many understand it as the shared life-breath that animates both humans and animals. Others think it leans closer to “spirit” language, stressing a common life-force while still leaving open questions about what happens beyond death.
3) How to take the question in v.21. Some read it as genuine uncertainty: the speaker says we cannot verify post-mortem outcomes. Others hear a sharper rhetorical edge: the question is meant to challenge overconfident claims and to keep the focus on what is observable (mortality), rather than to deny any possibility of difference.
Why the disagreement exists
The wording supports more than one emphasis. The passage moves from what can be seen (death, breath, dust) to what cannot be confirmed (“who knows…?”). Readers differ on how much v.21 is meant to suspend judgment about the afterlife versus simply restrain human certainty. Also, “breath/spirit” language overlaps in meaning, making it hard to draw strict lines.
What this passage clearly contributes
Ecclesiastes 3:18–21 presses a theological anthropology of humility: humans are not set above the rest of living creatures when it comes to mortality (explicit claim). It also frames knowledge limits as a spiritual and intellectual reality: the speaker refuses to present unverifiable claims as certainty (explicit claim). A further inference consistent with the passage is that God’s governance includes exposing human pretensions by confronting them with death’s leveling power (inference from “God tests them” plus the argument that follows).