Zettelkasten is the oldest concept here, and the deepest. The German word means "slip box." Its own definition is precise: "a personal tool for thinking and writing" with "hypertextual features to make a web of thoughts possible."
Niklas Luhmann was a sociologist who kept tens of thousands of small paper notes, each holding a single idea, each linked to others by hand. He published 50 books and over 600 articles, and said plainly that his productivity came "from working in a partnership with his Zettelkasten."
The unit is a single Zettel, one atomic thought, written in your own words, never copied verbatim. Each one has three parts.
[ID] a unique identifier, so the note can be linked and found
[Body] exactly one idea or concept, in your own words
[Refs] links to related notes, each with a note on WHY they connect
The IDs are what make it a web. Luhmann used a branching scheme like 1, 1a, 1b, 1a1, so a new thought could slot in right next to its parent. Many modern tools use a timestamp instead, like 202006110955. Either way, notes connect through explicit links, and the important detail is the link context, a short note on why two ideas belong together. That reasoning is where new knowledge gets made.
The popular on-ramp, from Sönke Ahrens' book How to Take Smart Notes, sorts your writing into three note types as a workflow.
Fleeting notes → quick captures, thrown away after you process them
Literature notes → what you took from a source, in your own words
Permanent notes → one finished atomic idea, linked into the web, kept
A word of caution. This one is for your thinking layer, your research and writing. It is overkill for your electric bill and your meeting notes. Not everything needs to be a permanent note.
GTD: the task system the others borrow from
Getting Things Done, by David Allen, is "a personal productivity methodology that redefines how you approach your life and work." It is the classic that most second brain systems quietly stand on.
Its core line is the entire philosophy. "Your mind is for having ideas, not holding them." You pull everything out of your head into a trusted system, so your brain can think instead of remember. The method runs on five steps.
- Capture. "Collect what has your attention." Everything, into one inbox.
- Clarify. "Process what it means." Is it actionable? What is the next action?
- Organize. "Put it where it belongs." Sort it into the right list.
- Reflect. "Review frequently." Keep the system trusted and current.
- Engage. "Simply do." Act with confidence from your lists.
The organizing step is where the structure lives. GTD keeps a handful of lists.
Next actions → the single next physical step, grouped by context (@calls, @computer)
Projects → anything that needs more than one action to finish
Waiting for → things you are blocked on, owned by someone else
Calendar → things that must happen on a specific day or time
Someday / maybe → not now, but you do not want to forget it
Here is the surprise. PARA's Projects and Areas came straight out of GTD. So if you adopt a second brain, you are already standing in David Allen's house. GTD is the piece to reach for when your knowledge is organized but your tasks are still floating in your head.
Diátaxis: the one for docs and how-tos
Diátaxis is the outlier here, and the one almost nobody in the second brain world mentions. It calls itself "a systematic approach to technical documentation authoring," and it solves three problems at once: what to write, how to write it, and how to organize it.
It says every document is one of four things, and you should never mix two of them in one place.
Practice-oriented Theory-oriented
Learning (study) Tutorials Explanation
Working (action) How-to guides Reference
Each quadrant has a clear job. A tutorial teaches a beginner by taking them through doing something. A how-to guide gives the steps to solve one specific real-world task. Reference is the dry, factual description you look things up in. Explanation is the background and the why. The two axes behind the grid are simple: theory versus practice, and acquisition (learning) versus application (doing).
This will not organize your whole life. What it will do is make every guide, checklist, and standard operating procedure you write dramatically clearer, because each one finally knows what job it is doing.
Which system is built for whom
These six are not really competitors. Each was built by someone solving their own specific problem, and their own words tell you who it is for.
- PARA was built for anyone, on any tool. Forte designed it to organize "any type of digital information across any platform," which is why it is the usual first step for beginners.
- Johnny.Decimal was built for "the life admin we all need to manage," and for teams and organizations where files pile up and finding is the bottleneck. Reach for it when your problem is sprawl.
- LYT and MOCs were built for note-makers, the people who want to develop ideas and think in an Obsidian-style linked vault, not just store files.
- Zettelkasten was built by an academic for writing. It suits researchers, writers, and anyone whose output is long-form and idea-heavy.
- GTD was built for personal productivity, for individuals drowning in tasks and commitments, with extensions for teams.
- Diátaxis was built for documentation authors and maintainers, for anyone who writes things other people will later have to read and follow.
Read that list as a menu, not a competition. Most people end up borrowing from more than one, because a task system, a filing system, and a thinking system are genuinely different jobs.
The part these concepts do not solve
There is one honest thing to say before you go and adopt any of them.
Every concept here organizes. None of them maintains. A framework will not move your finished project to the archive. It will not file the note you took at 2am. A perfect structure that you have to hand-feed every single day slowly turns into a museum, beautiful and untouched.
That is the gap I keep writing about. The structure decides where knowledge goes. Something still has to actually put it there and keep it current. For me that something is a set of AI agents built with Claude Code and the Hermes agent, doing the filing while I sleep. I wrote about that in At 52, my second brain finally works. The concept and the automation are two halves of the same tool.
Start with the concepts, though. They are the vocabulary. Once you can name what each one is for, you stop guessing and start choosing.
So here is my question for you. Of these six, which one have you already tried, and did it stick, or did it quietly become a museum?
