The famous detail, a slave in the triumphal chariot whispering respice post te, hominem te memento, look behind you, remember you are a man, comes from Tertullian's Apologeticus, a second-century Christian writer from Carthage who, as far as we know, never witnessed a Roman triumph. He never says slave; Jerome later says companion; Cassius Dio has the figure say only look behind you. Mary Beard's verdict: fragmentary, contradictory, possibly a Christian reinterpretation. The phrase memento mori itself is medieval.
The practice is real even if the slave is not: Seneca, let us balance life's books each day; Marcus Aurelius, you could leave life right now, let that determine what you do; Epictetus making the fear of death, not death, the enemy. Medieval Christianity industrialized it, skulls on desks, the Danse Macabre, vanitas paintings, until remembering death became Europe's standard operating system for humility.
The claim: the triumph story persists because power requires an invented witness, no institution can hire a real conscience, so it fictionalizes one, and every era re-casts the slave: the court jester, the free press, the anonymous quote-tweet. Verdict: Tertullian invented the witness for exactly this polemical purpose, deflating emperor-worship, so the myth's function is documented from birth. The question the file leaves open: who stands in your chariot, and did you appoint them, or did the crowd?
If the ritual is a myth, why has it disciplined egos for two thousand years?