The phrase “second brain” is glued to Tiago Forte and the productivity wave of the 2010s. But the ambition behind it is nearly five centuries old, and the method is a lot older than the name. So let’s walk the timeline, because the story is more interesting than the buzzword.
\nGo back to antiquity and you’ll find people keeping personal notebooks of quotes, recipes, and observations. The commonplace book. Nothing fancy, just a place to park the ideas worth keeping.
\nThings get more systematic in the mid-1500s. A naturalist named Conrad Gessner suggested cutting notes into individual slips and gluing them onto sheets so you could rearrange and reassemble ideas from pieces. That modular instinct, breaking knowledge into movable units, is the seed of what the Germans would later call the Zettelkasten, the “slip box.”
\nAbout a hundred years later, Thomas Harrison built the Arca Studiorum, the “ark of studies.” It was a literal cabinet where paper slips hung on labeled metal hooks, sorted by subject. The design was published posthumously by Vincent Placcius in 1689, which makes it one of the first documented personal knowledge devices. Leibniz reportedly relied on it for one of his projects. A hundred years after that, Carl Linnaeus was working with standard-sized paper slips, over a thousand of which survived. Basically the index card before the index card existed.
\nFor roughly 300 years this stayed a scholarly habit. Researchers, clergy, naturalists, the PhD crowd. Not something the general public thought about.
\nIn 1945, Vannevar Bush wrote an essay in The Atlantic called “As We May Think,” and described the Memex: a desk-sized microfilm machine that would store all of your books, records, and correspondence, with mechanical “associative trails” linking related items together. It never got built. But read that description again and tell me it doesn’t sound like every linked-notes tool we use today.
\nThen comes the patron saint of the second brain: Niklas Luhmann. From the 1950s onward he built a Zettelkasten of around 90,000 index cards over four decades. Each card got a unique ID, each linked to others by ID, a physical knowledge graph made of paper. Out of it came dozens of books (some counts say 70) and hundreds of articles. He described the system as a thinking partner he could have a conversation with. His archive was digitized and put online in 2019, so you can go poke around in it.
\nLuhmann wasn’t a one-off. The 20th century is full of scholars running the same playbook:
\nThrough the 1980s to the 2000s, we still didn’t have today’s vocabulary. We had the PIM, the personal information manager, and the PKM, personal knowledge management. The personal computer itself was pitched as the thing that would know everything about you.
\nApple, Xerox, and Microsoft all took a swing:
\nIn 2017, a writing coach named Sönke Ahrens published How to Take Smart Notes. He translated Luhmann’s dense German academic method into plain English for students and knowledge workers, and put the slip-box workflow, capture, permanent notes, link, develop, in front of a non-academic audience for the first time.
\nAlmost in parallel, Tiago Forte coined the modern “second brain” and aimed it squarely at the everyday knowledge worker. Through Forte Labs he taught PARA (Projects, Areas, Resources, Archives), an action-oriented filing system that rejects the Dewey Decimal instinct, and the CODE workflow (Capture, Organize, Distill, Express) for the life cycle of a note. The 2022 book Building a Second Brain turned it into a movement.
\nThen the tools caught up to Luhmann. Roam Research (2020) made bidirectional links the whole point. Its early adopters were overwhelmingly academics, PhD students, and writers, and they showed everyone what diligent linking actually buys you.
\nObsidian launched around the same time and is what most people picture now. Local-first, plain markdown, bidirectional links, a huge plugin ecosystem, and a motto that matters: your data is yours. That’s the real pitch. With Roam, Evernote, or Notion you can get your data in, but getting it back out in a format you own is a different story. You’re renting access to your own thinking. Obsidian doesn’t do that. It’s just markdown files on your disk. For my money that makes it the default, and everything with lock-in is a harder sell.
\nEither way, the shift is the headline: we moved from folders and notebooks to graphs. Links and backlinks. The knowledge graph stopped being a Luhmann eccentricity and became the norm.
\nWhich brings us to right now. Somewhere around 2023, people started asking the obvious question: what if you point a language model at your second brain? The best way I’ve found is through a CLI-based agentic harness like Claude Code, pointed at your notes. I’ve been running Claude Code against my own vault for over a year, and the same approach works with other agentic coders. OpenCode is good, and Google’s Antigravity (Gemini) has been pretty good too.
\nThere are different strategies for structuring a vault so an agent can read and extend it, and I’ll dig into those in a future post. One you may have heard of is Karpathy’s recent “LLM wiki” idea, which isn’t just about plugging an LLM in, it’s an opinion about how to structure the vault so the model works well with it.
\nThat’s the real evolution. For 500 years the second brain was storage you read from. Now it’s becoming something that reads and writes back.
\n| Era | \nMetaphor | \nKey figure/tool | \nWhat it solved | \n
|---|---|---|---|
| 1540s-1890s | \nCard file / commonplace book | \nGessner, Harrison, Linnaeus | \nModular scholarly notes | \n
| 1945 | \nMemex (associative trails) | \nVannevar Bush | \nThe idea of external memory | \n
| 1950s-1990s | \nZettelkasten (linked slips) | \nNiklas Luhmann | \nMemory as a thinking partner | \n
| 1980s-2000s | \nPIM (folders, notebooks) | \nHyperCard, OneNote, Evernote | \nDigital storage | \n
| 2010s | \nMethodology + capture | \nTiago Forte (BASB, PARA/CODE) | \nA repeatable workflow | \n
| 2020-22 | \nKnowledge graph | \nRoam, Obsidian | \nConnection over hierarchy | \n
| 2023- | \nAgent-readable wiki | \nLLMs, Claude Code, Karpathy | \nActive synthesis | \n
Gessner was gluing paper slips onto sheets so he could rearrange his ideas. We’re doing the same thing. We just gave the slip box a way to talk back.
\n\n\nI’d appreciate a follow. You can subscribe with your email below. The emails go out once a week, or you can find me on Mastodon at @logan@llbbl.blog.
\n