The Story So Far
Mechanistic interpretability is the science of “reading an AI’s mind”.
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interpretability → read → mind
Large language models are “grown, not built”.
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models → grow → ?
Researchers run training data through a neural network.
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Researchers → run → network
Eventually this creates a working AI; nobody really knows how.
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nobody → create → AI
But a neural network is just a set of simulated neurons on a computer.
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network → simulate → computer
The person with the computer can see the neurons, the connections between them, and which ones activate when the AI answers questions.
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AI → see → questions
So it seems like it should be possible to “reverse engineer” the AI.
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it → seem → AI
This would be scientifically useful: understand how AIs work, with possible relevance to human cognition.
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AIs → understand → cognition
It could also be practically useful: find the circuits responsible for dishonesty, hallucination, bias, and other negative behaviors, then redesign those circuits.
uncertain
It → find → circuits
A modern AI has millions of neurons and trillions of connections between them (aka “parameters”, “weights”).
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AI → have → them
Their structure is apparently nonsensical: researchers started by seeking a 1:1 mapping between neurons and concepts, like a neuron that always fired when the AI was thinking about cats, but quickly learned that nothing like that existed.
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nothing → start → that
Their first big breakthrough came in 2023.
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breakthrough → come → 2023
Rather than a 1-to-1 neuron-to-concept mapping, they discovered a many-to-many mapping.
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they → discover → mapping
Neurons 1 and 2 firing together might mean cat; 1 and 3 firing together might mean chair; 2 and 3 firing together might mean bread.
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2 → fire → bread
Such mappings let AIs with “only” tens of thousands of neurons represent millions of concepts.
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AIs → let → concepts
There were far too many of these combinations for humans to map, but researchers set other, smaller AIs to mapping them and met with some early success - like this functional combination of neurons (“feature”) representing “God” in an artificially-simple 512-neuron AI:
Spirits were high.
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Spirits → be → AI
Scale up the AI interpreter, and maybe we could do anything!
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we → scale → anything
Philosophers could discover the fundamentals of cognition.
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Philosophers → discover → cognition
Doomers could solve alignment by tracing the circuits involved in moral reasoning.
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Doomers → solve → reasoning
Tens of millions of dollars poured into the field.
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millions → pour → field
Superstar interpretability researcher Chris Olah got an audience with the Pope (okay, fine, this was mostly unrelated).
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this → get → Pope
By 2024-2025, the clarity of many-to-many mapping had dissolved, and everyone was back to feeling confused and miserable.
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everyone → dissolve → mapping
Strategies that had successfully analyzed artificially-simple research-subject AIs failed to work on real language models.
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that → analyze → models
Different teams making many-to-many maps of the same AI reported different results.
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teams → make → results
And humans watching supposedly-mapped AIs more closely found that seemingly-crisp features were much vaguer and worse than previous analysis had led them to believe: a feature which they thought meant “God” might activate on any vaguely religious or awe-inspiring text, or even text with no logical connection whatsoever.
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God → watch → connection
As for attempts to use the many-to-many maps for useful work - like AI lie detection - they generally failed to outperform simpler methods.
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they → use → methods
So researchers abandoned the hope of One True Technique - even if that technique was “actually understanding things” - and went back to the drawing board.
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technique → abandon → board
Now mechanistic interpretability is starting to feel exciting again - but also more intimidating than ever, as one tries to keep all the jargon straight.
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one → start → jargon
This list is my log and personal reference document for remembering what everything is.
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everything → remember → ?
It’s loosely inspired by the concept breakdown in Benjamin Sturgeon’s Contributing To Technical Safety Research In The AI Safety End Game, which itself follows the Claude Mythos Preview System Card.
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which → inspire → Card
Imagine an AI with three neurons: [0, 0, 0]
The researcher makes the AI think of cats in various contexts, averages them all out, and gets some set of activations like [0.8, 0, 0.35].
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AI → imagine → ]
Imagine the AI as operating in a Cartesian space where each of its neurons is one dimension.
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each → imagine → neurons
The vector from [0, 0, 0] to [0.8, 0, 0.35] defines a direction in that space.
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vector → define → space
In a linear probe, researchers compare the true direction of the AI’s thoughts to one of these pre-defined concept-representing direction vectors.
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researchers → compare → vectors
Because activation-space has thousands of dimensions, its behavior is counterintuitive, and a particular thought can be in thousands of “directions” at once and yet still “closer” to some directions than others.
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thought → have → others
Experiments confirm that when an AI’s thoughts are close to the cat direction, this is a good sign that it’s thinking about cats.
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it → confirm → cats
Why doesn’t this solve the whole problem?
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this → solve → problem
First, this teaches us very little about how AIs work - how, for example, they might answer a difficult query in veterinary medicine.
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they → teach → medicine
At best, it’s a primitive lie detector: if an AI claims it isn’t thinking about cats, you can check its work (either by probing the concept of cat, or by probing the concept of lying!)
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you → ’ → lying
Second, it’s only as good as the original linear probe.
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it → ’ → probe
…and 165 more, not listed.
