Chapter 3 addresses the gap between “knowledge in the head” (what people know) and “knowledge in the world” (explicit design cues), arguing it is the designer’s job to bridge this gap. Norman holds that people rarely need to understand the details of complex theories or inventions—simplified approximations generally suffice.

Precise Behavior from Imprecise Knowledge

People derive information from two sources: their minds and external cues. If designers put sufficient cues into their products (“knowledge in the world”), they can prompt users who have imprecise knowledge about the product to behave in precise ways. People do not need an in-depth understanding of complex ideas and inventions, just enough to complete tasks. Norman describes two kinds of knowledge: knowledge of (declarative knowledge) and knowledge how (procedural knowledge). The former includes knowledge of rules and facts, while the latter is difficult to write down and best taught by demonstration.

Research in the field of psychology reveals that people only retain partial information because external knowledge makes precise knowledge unnecessary (80). Signifiers and mappings are perceivable cues that function as external knowledge. Norman explains that this type of knowledge is everywhere, from letters on keyboards to labels on controls. Constraints also allow people to function with incomplete knowledge.