What is architectural decision records?

Architectural decision records is a concept in personal epistemology: The most valuable thing to capture is why you chose what you chose. Decisions decay faster than facts — and unlike facts, they cannot be reconstructed after the outcome is known.

Example: A team lead chooses PostgreSQL over DynamoDB for a new service. Six months later, the service struggles under write-heavy load. Everyone says 'obviously we should have used DynamoDB.' But the original decision was made because the team had zero DynamoDB experience, the deadline was eight weeks out, and the data model was deeply relational. Without a written record of those constraints, the team learns nothing — they just blame the decision-maker. With a decision log, they can ask the right question: 'Given what we knew then, was the process sound? What new information would change the answer?'

This concept is part of Phase 10 (Externalization Mastery) in the How to Think curriculum, which builds the epistemic infrastructure for externalization mastery.