On the Biology of a Large Language Model (Anthropic, 2025) LLMs are not the "black box" you were promised. Mechanistic interpretability — peering into a neural network to reverse engineer its inner workings — has made major strides. Anthropic's On the Biology of a Large Language Model (2025) is a landmark in that effort. What follows is a summary of their progress and some related thoughts. What is an LLM actually "thinking"? How can we understand what an LLM is "thinking"? It's clearly very val...