Jevons closed his exposition of 'indirect inference' in Pure Logic (1864) by quoting Chrysippus's three-road dog — in Sir Walter Raleigh's wording, not the ancient Greek

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Section 136 of William Stanley Jevons's Pure Logic (1864) closes his account of what he calls indirect inference — his term for reasoning by elimination, the disjunctive-syllogism move — with a verbatim quotation of the Chrysippus three-road dog, offered as evidence that "even brute animals... may reason by the indirect method." In Jevons's printed text the dog reasons that "the Beast must be gone either this, or this, or the other way; but neither this nor this; Ergo, the third: so away he runneth."

Two facts about the quotation matter. First, it settles the long-open question of whether Jevons engaged the dog at all: he did, in his own hand, at the hinge of his first logical treatise — the disjunctive-syllogism example that the vault's reasoning-by-elimination through-line had connected to Jevons only as a structural echo. Second, Jevons does not quote it from an ancient source. His copy is printed word-for-word as Sir Walter Raleigh's rendering, credited "— SIR W. RALEIGH." at the close — a transmission distinct from the Sextus Empiricus / Outlines of Pyrrhonism I.69 line that grounds the vault's Chrysippus note.

What the quotation does not establish is that Jevons treated the dog as a model for mechanizing inference. It illustrates the logical form; it names no machine. The claim that Jevons saw the dog as an example specifically for mechanization is a later historian's reading, not this passage (claim-jevons-dog-as-mechanization-model-is-barrett-connell-interpretation), and Jevons's own machine-justifying paper runs the other way (claim-jevons-1870-paper-credits-boole-not-ancients-for-mechanizable-logic).