You already think outside your head.
You count on your fingers when the numbers get awkward. You write a shopping list so you can stop rehearsing it. You put a dentist appointment in a calendar in order to forget it on purpose. None of that is cheating. It is how thinking has mostly worked — a bit in your head, a bit in the things around you.
The idea has a name, and a ship.
Researchers call it distributed cognition. The anthropologist Edwin Hutchins made the case for it in Cognition in the Wild (MIT Press, 1995), built on fieldwork with the navigation teams that bring large US Navy vessels into port.
Nobody on that bridge is doing the navigation. Sailors out on the wings sight landmarks and read off bearings. Someone relays them over a phone circuit. Someone else plots them on a chart with a straightedge, and where the lines cross is where the ship is. The chart, the instruments and the standing procedure carry as much of the work as any of the people do. The ship's position gets computed, but no single head computes it. The thinking unit is the whole bridge.
That is a strange thing to read about a ship and an obvious thing to notice about a kitchen. A calendar on the fridge, a whiteboard, a note left by the kettle: small pieces of a household's memory that live outside anyone's skull.
Otto and his notebook.
Three years after Hutchins, the philosophers Andy Clark and David Chalmers published “The Extended Mind” in Analysis, and gave the idea its most-quoted example. Inga hears about an exhibition, remembers the museum is on 53rd Street, and walks there. Otto has Alzheimer's; he carries a notebook everywhere, looks up 53rd Street, and walks there too. Clark and Chalmers argue that the notebook is not a substitute for Otto's memory. For all the purposes that matter, it is his memory.
What makes the argument useful is that they say exactly when it holds. The notebook has to be “a constant in Otto's life” — he rarely acts without consulting it. Its contents have to be “directly available without difficulty”. And on reading it, he has to automatically endorse what he finds. Always there, easy to consult, believed without a second check.
This is philosophy, not a lab result, and people have been arguing about the metaphysics ever since. You don't have to settle any of it. Those three conditions are worth stealing on their own, because they are also the difference between a list that works and a list that nags.
GTD is this, applied to promises.
Getting Things Done is not a philosophy book and doesn't cite either of these. David Allen got to the same place from the other end — from watching people drown in commitments — and said it in one line.
Your head is for having ideas, not holding them.
— David Allen, Getting Things Done
The whole loop follows from that. Capture everything, so nothing is left circling. Clarify each item down to a next action, because a vague note can't guide anything. Organize the results into a few lists you can believe. It is a procedure for moving your obligations out of the one storage device that is bad at storing them, and into something built for the job.
An external mind has requirements.
Otto's notebook works because it meets four conditions. Miss one and your head takes the job back, and the low-grade hum of “I'm forgetting something” starts up again.
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Always within reach
Ideas arrive on the bus, in the shower, mid-conversation. If your system isn't there at that moment, the thought stays in your head, and now you are holding it and the guilt of not writing it down.
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Complete
Two lists means neither is the list. Your mind can tell when something has been left out, so it keeps its own shadow copy of everything — the worst of both arrangements.
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Trusted without checking
The moment you half-believe a list, you start double-checking it, and the second check is the expensive part. You trust a list because it was right last time, and the time before.
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Reviewed on a schedule
Nothing external maintains itself. This is the whole reason the Weekly Review exists: twenty quiet minutes to finish what's done, drop what's dead, and make the list true again. Skip it for a month and the list becomes a museum.
A rented mind is a fragile one.
If part of your memory lives in an app, that app's terms are your memory's terms. Subscriptions lapse. Servers go down at the wrong moment. Companies get acquired and sunset the product with six months' notice, and you discover the export you never tested doesn't contain what you assumed.
That is the gap Mindwtr was built into. Your tasks live in a SQLite file on your own disk — one of the most widely used file formats there is, readable without us. It works offline by default and needs no account. Sync is opt-in when you want it, over iCloud, Dropbox, WebDAV or a plain folder, with an optional passphrase that encrypts the files before they leave your device — so even the storage provider can't read them. And it is free and open source under the AGPL-3.0, which means there is no billing date on which your external mind stops working.
None of that makes it a better place to think. It makes it a place that will still be there.
Keep reading
If this is your kind of question, carry on with the psychology of GTD, the Zeigarnik effect and why unfinished things nag, and the Mindwtr Handbook — a fifteen-minute tour of the whole system in practice.