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THE mind-body problem is about where the mind is located. Is it bound by the confines of our skull or does it lurk nearby, non-physically?

The latter idea, that the mind is not literally in space but is located 鈥渘ear to鈥�, or even 鈥渞ight next to鈥� the body 鈥� a halo-like master of ceremonies circumnavigating the head 鈥� is metaphysically weird but popular. As recently as 1949, the philosopher insisted that this notion of the 鈥済host in the machine鈥� was the official view among most laypersons, scientists and intellectuals.

How a mind lacking mass, longitude and latitude could be 鈥渋n鈥� or 鈥渘ear鈥� the particular body whose experiences it is supposed to have, and whose actions it is supposed to initiate, remains mysterious. Still, despite a certain occult je ne sais quoi feel, this view seems intuitive in a plebeian, man-on-the-Clapham-omnibus way. When I try to remember how to get to New York鈥檚 Museum of Modern Art from the corner of Broadway and 42nd Street, it seems as if my mind asks my brain, which is nearby, and my brain, which stores addresses, fetches the address and passes it to 鈥渕e鈥� (鈥淚鈥� live where my mind lives, not where my brain is housed). Let鈥檚 call this picture of the mind鈥檚 whereabouts Nearby: the mind is not in the brain or in the body, but it is close.

By the mid-1950s, though, philosophers with apocryphal-sounding surnames like (Jack) and (Ullin) had articulated an alternative to this unstable view: the mind is the brain. The mind isn鈥檛 in the brain in the metaphorical way that my beloved has a 鈥減lace鈥� in my heart. The mind is literally, physically in the brain because, well, that鈥檚 where it is and that鈥檚 where it was all along. When I try to remember where the MoMA is, my brain鈥檚 prefrontal cortex initiates a 鈥渇etch鈥� request to my memory, which passes the information back to the prefrontal cortex, which in turn slides the retrieved info over to the motor cortex, and I am on my way.

In his brilliant new book, Supersizing the Mind, calls this view, which holds sway among the neurocognoscenti, Brainbound. Even though Nearby still has the mother lode of fans among the hoi polloi, the problem of the mind鈥檚 location has, at least among the experts, been solved. Mind is Brainbound.

Or so it seemed. Now a few edgy cognitive scientists and philosophers, including Clark and , claim that both Nearby and Brainbound get the location wrong. There is a third place where the mind might roam. The theory Clark calls Extended says that the mind is in space, as all sensible naturalists claim, but is smeared over more than brain space. 鈥淐ertain forms of human cognizing include inextricable tangles of feedback, feed-forward and feed-around loops: loops that promiscuously criss-cross the boundaries of brain, body and world,鈥� he writes.

Think about it: when you walk to the MoMA, the you-walking-to-the-MoMA is cognitive, intentional and mindful. But the walking is not in your head 鈥� it extends into the world. Walking, talking and seeing are all things the enactive, embodied, extended (code words for this hip new view) mind does in the world. Ironically, dualists, the advocates of Nearby, got the smeariness of mind right. Their mistake was thinking that the smeariness and neariness of mind meant that mind was in no space at all, metaphysically out of this world. According to Extended, the mind is in space and explained entirely by physical processes, but is not confined to the brain.

The thesis of the extended mind, according to philosopher who offers a terrific introduction to Clark鈥檚 book, is that 鈥渨hen parts of the environment are coupled with the brain in the right way, they become parts of the mind鈥�. Suppose Owen is reliably challenged when it comes to remembering addresses, so he always writes important ones down in his notebook. Owen is in Times Square on his way, again, to the MoMA. When he forgets its address, he consults his notebook in just the way others would activate a 鈥渇etch鈥� request to memory. Extended says that the notebook 鈥� or the Owen-notebook coupling 鈥� counts as part of the cognitive process that reliably results in Owen鈥檚 getting to where he wants to go. The Owen-notebook coupling is functionally equivalent to the Owen-internal memory coupling that Owen might have used, once did use, and so on. The idea ramifies.

Is Extended true? No毛 is convinced. His little book Out of Our Heads is a desultory hodgepodge of occasionally interesting but perfectly familiar ideas: that the mind is an active rather than passive spectator, that we are fully embodied, and that the mind has all sorts of causes and effects outside of the body.

Armed with these plausible ideas, No毛 takes aim at Brainbound. He writes: 鈥淭he fundamental assumption of much work on the neuroscience of consciousness is that consciousness is, well, a neuroscientific phenomenon. It happens inside us, in the brain.鈥� No毛, instead, promises to show that 鈥渃onsciousness does not happen in the brain鈥�. But he never even tries to show such a thing, nor could he if he had tried.

The thesis that consciousness 鈥渄oes not happen in the brain鈥� is a much stronger and less plausible thesis than Extended, which Clark is careful to say is a thesis about cognition, thought and mind 鈥� not about experience. Clark is careful in a way No毛 is not with the consciousness-cognition distinction. Cognition is about thought processes; consciousness is about experience. And experience, as far as we know, is Brainbound or at least 鈥渙rganism-bound鈥�. Experiences, at the moment anyway, supervene only on bodies that house biofuelled nervous systems. But cognition already roams more widely. In our age of cognizing machines, the reach of cognition, outside small circles of homo- or animal-chauvinists, is known: cognition extends beyond human and animal minds.

Once we entertain the Extended thesis, likely suspects of extended cognition begin to abound. Take the fascinating research on gesturing, for instance, which suggests that gesturing is not only causally relevant to thoughts, but constitutive of the thinking process itself.

Supersizing the Mind provides the best argument I鈥檝e seen for the idea that minds are smeared over more space than neuroscience might have us believe, and that mind will continue spreading to other nooks and crannies of the universe as cognitive prostheses proliferate.

鈥淢inds are smeared over more space than neuroscience would have us believe鈥�

The pay-off for thinking in accordance with Extended is a better understanding of mind and a roomier division of intellectual labour among cognitive scientists. 鈥淭o unravel the workings of these embodied, embedded, sometimes extended minds, requires an unusual mix of neuroscience, computational, dynamical, and informational-theoretic understandings, 鈥榖rute鈥� physiology, ecological sensitivity, and attention to the stacked designer cocoons in which we grow, work, think, and act,鈥� Clark writes.

The lesson is that despite a consensus that the mind is in the world, we don鈥檛 yet know exactly where in the world it is. Still roaming after all these years.