PARRY and ELIZA/DOCTOR were judged by the same indistinguishability test while running on opposite mechanisms — simulated internal affect-states versus keyword transformation with no modelled beliefs or affect

imitation-gameturing-testelizaparrymechanismhistory-of-csindistinguishability-test

The two best-known 1960s–70s conversational programs — Kenneth Colby's PARRY and Joseph Weizenbaum's ELIZA (its DOCTOR script) — were held to the same evaluation standard, an indistinguishability criterion, despite opposite answers to the question of what sits between input and output: a modelled mind, or nothing.

Colby, Weber & Hilf's 1971 "Artificial Paranoia" describes PARRY as an explicit belief-and-affect simulation: "It is assumed that the detection of malevolence in an input affects internal affect-states of fear, anger and mistrust, depending on the conceptual content of the input." Those three variables persist and decay across turns, and the reply is chosen under their control — "the actual type of response usually depends only on a simple check of the affective context of the interview." The same paper states the test explicitly: "Our model is testable by means of indistinguishability tests. If the model's paranoid I-O behavior cannot be distinguished from its human counterpart by psychiatric judges using a diagnostic interview, then we shall consider the simulation to be successful" — Colby's own psychiatric-register name for Turing's imitation-game criterion.

The opposition is not in the input machinery: Colby's program has no natural-language parser either and "relies on key-word understanding." Both scan keywords. What differs is what the keywords reach.

In ELIZA they reach a rule table and stop. Weizenbaum's 1966 paper: "Input sentences are analyzed on the basis of decomposition rules which are triggered by key words appearing in the input text. Responses are generated by reassembly rules associated with selected decomposition rules." Nothing represents what the speaker believes or what the program feels. It is not, however, stateless: the unabridged CACM text gives a reserved pseudo-keyword MEMORY whose transformation of an earlier input "is stored on a first-in-first-out stack for later use," replayed turns later when an input carries no keyword — the mechanism behind the closing line of Weizenbaum's own sample dialogue, which returns to a boyfriend mentioned twelve exchanges earlier. The contrast with PARRY is modelled inner states versus none, not memory versus no memory.

Both nonetheless held up their end of a real 1972 ARPANET conversation well enough to be published verbatim — RFC 439. The shape is therefore not the vault's inference about that transcript: it is what both primary papers say about their own programs. It recurs one level down, inside a single modern model — observation-same-output-opposite-mechanism-recurs-eliza-parry-to-llm-circuit.