A mind palace is any system built from scratch by a person or a small group, whose builder relies upon it as a cognitive lever. It has two parts: a mental model and an externalized implementation.
A spider’s web is a mind palace. A stack of books and notebooks can be a mind palace. Your smartphone is a mind palace.
Computers are unreasonably effective as tools for building mind palaces.
In The Extended Mind, Andy Clark and David Chalmers argue that the environments we build around ourselves deserve epistemic credit for the ways in which we use them to facilitate our thinking. Looking up a fact we have written in a notebook, they argue, is not different from recalling it by introspection. Going further: if you write down something you believe to be true in your notebook, then as long as you rely on that notebook you can still be said to hold that belief, even if it disappears from your immediate mental recall!
Clark and Chalmers are concerned with whether and how a mind or self can extend into one’s environment; I take it for granted that they do. I am more interested in the resulting systems themselves, so I need a term for them. Mind palace serves well, I think (despite possible confusion with “memory palace” or method of loci): it is designed, crafted, and out in the world — and it is a part of your mind.
Because mind palaces bear some amount of a person’s mental load and processes, a disruption to their mind palace constitutes actual violence to their mind. This is a useful way of understanding (for example) the angry (and uneven) responses people have to software updates, or the mental impairment caused by moving to a new home, even voluntarily.
The character of Mr. Norell in the book Jonathan Strange and Mr Norrell neatly illustrates the appeal and the problem of mind palaces. In practice, he relies on a particular mind palace (his library at Hurtfew Abbey) to an absurd degree; he has spent years cataloguing and indexing his books, and nearly all his conscious beliefs are held with reference to the things contained there. When he moves temporarily to his home in Hanover Square, his sole and overwhelming concern is that only part of the library can accompany him. But as in so many other areas, his stated opinions do not agree with his practice:
Mr. Norrell was speaking. “Many magicians,” he said, steepling his hands, “have attempted to confine magical powers in some physical object. It is not a difficult operation and the object can be any thing the magician wishes. Trees, jewels, books, bullets, hats have all been employed for this purpose at one time or another.” Mr Norrell frowned hard at his fingertips. “By placing some of his power in whatever object he chuses, the magician hopes to make himself secure from those wanings of power, which are the inevitable resut of illness and old age. I myself have often been severely tempted to do it: my own skills can be quite overturned by a heavy cold or a bad sore throat. Yet after careful consideration I have concluded that such divisions of power are most ill-advised. Let us examine the case of rings. Rings have long been considered peculiarly suitable for this sort of magic by virtue of their small size. A man may keep a ring continually on his finger for years without exciting the smallest comment — which would not be the case if he shewed the same attachment to a book or a pebble — and yet there is scarcely a magician in history who, having once committed some of his skill and power to a magic ring, did not somehow lose that ring and was put to a world of trouble to get it back again.”