The 'Third Man Factor' name descends from T.S. Eliot's The Waste Land, whose own note hedges about its Antarctic source

The popular and clinical name for the feeling-of-a-presence hallucination (claim-robot-induced-feeling-of-a-presence-models-patient-hallucination) — the "Third Man Factor" — is borrowed from modernist poetry, not coined from neuroscience. Its lineage runs through T.S. Eliot's The Waste Land (1922), whose line 360 asks, "Who is the third who walks always beside you?" Eliot's own footnote credits an Antarctic-expedition memoir while openly hedging about which one: "The following lines were stimulated by the account of one of the Antarctic expeditions (I forget which, but I think one of Shackleton's)… the party of explorers, at the extremity of their strength, had the constant delusion that there was one more member than could actually be counted."

The source Eliot half-remembers is Ernest Shackleton's memoir South (1919), in which Shackleton wrote that during the final crossing "it seemed to me often that we were four, not three." The poem quietly changed that four into a third walking companion; the modern term descends from Eliot's reformulation rather than directly from Shackleton's own words.

The recursion is the interesting part, and it links this note to the mechanism cluster: an exhausted explorer's brain fabricates an extra member; a poet decades later mis-remembers the account and alters the headcount; and a laboratory decades after that reproduces the same "extra person" sensation on demand (claim-angular-gyrus-stimulation-induced-out-of-body-experience). Memory distortion is the mechanism at every layer, not only the neurological one.

Sourcing note: Eliot's published note is quoted here verbatim, but the load-bearing etymological claim — that the scientific/clinical term descends specifically from the poem — currently rests on tertiary sources and is a mildly surprising historical claim; under the vault's sourcing floor it is flagged and routed for primary verification via question-verify-third-man-factor-clinical-term-eliot-etymology. Scholars separately note an unremarked biblical layer (the Road to Emmaus, Luke 24), where two travelers are joined by an unrecognized third.