I'm not suggesting that the mind is non-physical or doubting that the brain is central to it; but it could be that (as Clark and others argue) the mind extends beyond the brain.

In writing, we are not simply recording our thinking but doing the thinking.
Clark and fellow philosopher of mind David Chalmers propose what's since been called the Parity Principle, which says that if an external artefact performs a function that we would regard as mental if it occurred within the head, then the artefact is (for the time being) genuinely part of the user's mind.

You might want to ask why we should think of minds extending into bodies and artefacts, rather than merely interacting with them. Does it make any difference? One answer is that, in the cases described, brain, body and world are not acting as separate interacting systems, but as a coupled system, tightly meshed by complex feedback relations, and that we need to look at the whole in order to understand how the process unfolds. (It's worth noting, too, that the brain itself is a collection of coupled subsystems.)

Language is a particularly powerful means of extension and enhancement, serving, in Clark's phrase, as scaffolding that allows the biological brain to achieve things it could not do on its own.

The brainbound view pictures the brain as a powerful executive, planning every aspect of behaviour and sending detailed instructions to the muscles. But, as work in robotics has illustrated, there are more efficient ways of doing things, which nature almost certainly employs.

Linguistic symbols provide new focuses of attention, enabling us to track features of the world we would otherwise have missed, and structured sentences highlight logical and semantic relations, allowing us to develop new, more abstract reasoning procedures (as in long division).

Similarly, rather than passively gathering information to construct a detailed internal model of the external world, it is more efficient for the control system to keep actively probing the world (to 'use the world as its own model', as the roboticist Rodney Brooks puts it), gathering just enough information at each step to advance the task at hand.

As the philosopher of mind Daniel Dennett notes, many elderly people are in Otto's position, relying on a host of cues around the home to guide them through their daily routines, reminding them of what to do, and when, and how.