Completeness means representing every fact relevant to the question at hand in a mental model. Because no model captures everything, relevance depends on the purpose: a circuit model may predict current yet omit the luminous efficiency needed to predict brightness. section 1.1.1 section 1.1.3
Figure 1.1 Mental model and system
Use the system-and-model illustration to compare ground truth with the facts represented in a mental model. section 1.1.1
Transactions hide distributed-system facts
A money-transfer handler visibly subtracts from one account and adds to another, but it need not display concurrency or failure handling when transactional guarantees provide those protections. Undo records operations before execution, enabling either a complete commit or an abort that restores the prior state. section 5.2 section 5.3.3





