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In the movie Memento, the hero, Leonard, suffers from a form of anterograde amnesia that results in an inability to lay down new memories. Nonetheless, he sets out on a quest to find his wife’s killer, aided by the use of notes, annotated polaroids, and (for the most important pieces of information obtained) body tattoos. Using these resources he attempts to build up a stock of new beliefs and to thus piece together the puzzle of his wife’s death. At one point in the movie, a character exasperated by Leonard’s lack of biological recall shouts: “You know? What do you know. You don’t know anything. In ten minutes’ time you won’t even know you had this conversation!” Leonard, however, believes that he does, day by day, come to know new things. But only courtesy of those photos, tattoos, tricks and ploys. Who is right? These are the kinds of question addressed at length in the essay (coauthored with David Chalmers) “‘The Extended Mind’” (1998 and this volume ). Is the mind contained (always? sometimes? never?) in the head? Or does the notion of thought allow mental processes (including believings) to inhere in extended systems of body, brain, and aspects of the local environment ? The answer, we claimed, was that mental states, including states of believing, could be grounded in physical traces that remained firmly outside the head. As long as a few simple conditions were met (more on which below), Leonard’s notes and tattoos could indeed count as new additions to his store of long-term knowledge and dispositional belief. In the present treatment I revisit this argument, defending our strong conclusion against a variety of subsequent observations and objections. In particular, I look at objections that rely on a contrast between the (putatively ) intrinsic content of neural symbols and the merely derived content of external inscriptions, at objections concerning the demarcation of 3 Memento’s Revenge: The Extended Mind, Extended Andy Clark 44 A. Clark scientific domains via natural kinds, and at objections concerning the ultimate locus of agentive control and the nature of perception versus introspection . I also mention a possible alternative interpretation of the argument as (in effect) a reductio of the very idea of the mind as an object of scientific study. This is an interesting proposal, but one whose full evaluation must be left for another time. First, though, it will help to briefly review the original argument from Clark and Chalmers (1998 and this volume). 1 Tetris and Otto Two examples animated the original essay. The first involved a human agent playing the arcade game Tetris. The human player has the option of identifying the falling pieces (1) by mental rotation or (2) by the use of the onscreen button that causes the falling zoid to rotate. Now imagine (3) a future human with both normal imaginative rotation capacities and also a retinal display that can fast-rotate the image on demand, just like using the rotate button. Imagine too that to initiate this latter action the future human issues a thought command straight from motor cortex.1 Now let us pump our intuitions. Case (1) looks, we argue, to be a simple case of mental rotation. Case (2) looks like a simple case of nonmental (merely external) rotation. Yet case (3) now looks hard to classify. By hypothesis , the computational operations involved are the same as in case (2). Yet our intuitions seem far less clear. But now add the Martian player (case 4) whose natural cognitive equipment includes (for obscure ecological reasons) the kind of biotechnological fast-rotate machinery imagined in case (3). In the Martian case, we would have no hesitation in classifying the fast rotations as a species of mental rotation. With this thought experiment as a springboard, we offered a parity principle as a rule of thumb: The Parity Principle If, as we confront some task, a part of the world functions as a process which, were it to go on in the head, we would have no hesitation in accepting as part of the cognitive process, then that part of the world is (for that time) part of the cognitive process.2 The parity principle invites us to treat the players’ use of the external rotate button, the cyberpunk implant, and the Martian native endowment as all on a cognitive par. But of course there are differences. Most strikingly , in case (2) the fast-rotate circuitry is located outside the...

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