The fourfold division in Plato’s Philebus develops machinery to decide the contest between pleasure and reason for second place after the mixture in the debate about the good life. The machinery consists of the limit, the unlimited, the mixture of limit and unlimited, and cause of the mixture. Plato’s Socrates collects instances of the unlimited and mixture to determine their unified nature and marks off the cause from the other three kinds and argues for its priority. We gain understanding of the limit by considering its operation in relation to the other three kinds. The limit is responsible for external and internal boundaries in an unlimited, bringing definiteness either by marking something off from things outside it or by providing internal structure. Chapter 5 argues that one key function of the limit is omitted in the fourfold division: guaranteeing the goodness of a mixture.