Daniel Kahneman's framework from "Thinking, Fast and Slow." System 1 is fast, intuitive, automatic — the pattern-matcher. System 2 is slow, deliberate, effortful — the reasoner. Most product decisions get made with System 1 and then rationalized with System 2; good founders learn to notice which system is actually driving the call. Directly relevant to UX design (the customer is mostly in System 1) and to self-awareness about when you're guessing vs thinking.