Physical clocks answer "what time did this happen?" Logical clocks answer a different question: "what could have caused this event?"

That distinction is the reason logical clocks exist. Many correctness problems in distributed systems do not require calendar time. They require causal order: knowing which events could have influenced which others, even when the participants disagree about the current time.

This chapter covers happens-before, causal vs total order, concurrent events, what logical clocks guarantee, and a short tour of Lamport timestamps, vector clocks, and hybrid logical clocks.