“What if not Circuits?” by CarolusRenniusVitellius

This post was written as part of the Iliad Fellowship. Inspired by conversations with Richard Ngo, Dmitry Vaintrob, and Brianna Grado-White. To all of these, my thanks.

Preface: I'm confused about how neural networks do and learn computations. In response to a friend's challenge, I'm writing up some interim thoughts. This essay has four parts: the first tries to track what I call the 'default ontology' of the mechinterp community over the years. The second part is about 'representational drift' as an important obstacle to weights-based approaches to circuits. The third part reflects on how 'universality' should shape our explanations of LLM function. The fourth part is a sketch of a 'co-selectionist' view of circuits I have been thinking about. These parts share a common theme but should be readable separately.

I want to understand how neural networks, LLMs in particular, work. In my research I've spent a lot of time trying to think through what kinds of explanatory accounts are best suited to this. In thinking about comparisons between evolution, neuroscience, and deep learning, I've ended up with an intuition like the following:

Large-scale learning processes like deep learning or the brain are different in [...]

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Outline:

(02:31) 1. What Might We Mean By "Circuits"?

(02:36) 1.A. Definitions

(04:57) 1.B. Circuits, Features, and MechInterp

(11:00) 2. The Central Problems of Noise and Representational Drift

(11:29) 2.A. Representational Drift in the Brain

(14:19) 2.B. Representational Drift in Neural Networks

(17:57) 3. Developmental Motifs Circumscribe Notions of 'Circuits'

(18:03) 3.A. Neurotrophins and Microstructure

(20:41) 3.B. Back to Neural Networks

(22:42) 4. A Co-Selectionist View: Circuits Move Together?

The original text contained 12 footnotes which were omitted from this narration.

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First published:
September 21st, 2026

Source:
https://www.lesswrong.com/posts/mMERyrvEJ4xbiozie/what-if-not-circuits

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Narrated by TYPE III AUDIO.

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