“Build a second brain” is easy to nod at and hard to start, because the obvious next question has no obvious answer: what do you actually put in it? Dump everything and you have a worse search engine. Put in too little and the answers stay generic. The useful middle is narrower and more specific than most people expect.
Five kinds of context earn their place.
1. How you write
Not a style guide - actual examples. Three or four pieces of your own writing that you were happy with, in the forms you produce most: the client update, the board note, the difficult email. A model shown four real examples of your voice will match it better than one given a page of adjectives describing it.
Include the ones with edits visible if you have them. Seeing what you cut is more instructive than seeing the final version.
2. What good looks like
The standard you judge work against, written down. This is the one people skip, and it is the one that makes a reviewer step possible: a workflow can only check a draft against a standard if the standard exists somewhere other than your head.
It rarely takes long. What must a proposal always contain? What makes a summary useless to you? What would you send back? Half a page of that outperforms any amount of general instruction.
3. The decisions you have already made, and why
The reasoning behind your positions - why you price the way you do, why you stopped offering that service, which kinds of client work out and which do not. Most AI advice is generic because the model has no access to the thinking that makes your situation yours.
The “why” matters more than the “what”. A decision without its reason cannot be applied to a new case; with it, the same judgement can be reused.
4. The recurring facts
The things you would otherwise retype every week: who your people are and what they do, your products and what distinguishes them, your constraints, your calendar shape, the acronyms your industry uses that a general model will read wrongly.
Dull to write, disproportionately useful. This is the layer that stops you spending the first third of every conversation explaining yourself.
5. The shape of the work you repeat
For each task you do regularly: what triggers it, what inputs it needs, what the output looks like, and where it usually goes wrong. This is also the raw material for any workflow you build later - the two things compound, which is why they suit two days rather than one.
What should not go in
- Credentials, keys and passwords. Never, in any form.
- Personal data about other people, unless your organisation has cleared the tool for it. Someone else’s information is not yours to relocate.
- Anything you would not put in an email to an external supplier. A useful test, and a fast one.
- Everything else you have ever written. A second brain is a curated supply, not an archive. Volume without curation is how these systems get quietly abandoned - the value is in what you left out.
The part that decides whether it survives
A second brain decays. Standards change, people move, decisions get superseded, and a store that silently goes stale is worse than none because you stop being able to trust it. So the maintenance routine is not an afterthought to the build: what gets reviewed, how often, and what triggers an update.
That is why building one is a day’s work rather than an afternoon’s, and why the day ends with the routine rather than the contents. The Oxford Second Brain Bootcamp does exactly that: you build yours from your own material, organise it so it stays findable as it grows, and leave with the routine that keeps it worth having.