river
Explanation
The comic depicts two ancient Greek philosophers standing by a river, referencing the famous fragment attributed to Heraclitus: "No man steps in the same river twice." In the first panel, one philosopher states this maxim, and the other responds "I do!"
The philosopher then demonstrates: "Look, I'm putting my foot in this river one time." Then: "Now I'm doing it a second time." The first philosopher objects: "But is it the same river?" The demonstrator pauses, then says: "Now, boy do I feel stupid." The final panel shows a separate character watching and commenting: "Just some Greek philosophers getting into a heated debate over Heraclitus."
The comic plays on Heraclitus's famous philosophical observation that you cannot step into the same river twice because the water is constantly flowing and changing -- therefore it is never the "same" river. The humor comes from the second philosopher initially rejecting this as pretentious nonsense ("I do!") and trying to disprove it through brute empirical demonstration, only to be caught by the actual philosophical point. His deflation from confident empiricist to "now boy do I feel stupid" mirrors the experience many people have with Heraclitean philosophy: it sounds like nonsense until you actually think about it.
The final panel, showing observers watching the argument, adds a meta-layer of comedy -- to outsiders, the sight of two grown men arguing intensely about whether a river is the same river looks absurd, which is itself a commentary on how philosophy appears to non-philosophers.