You have already seen consensus (T9) in Book 4, where distributed nodes agree on a value through a defined protocol. Tests are consensus at code level: they express agreement between the specification (what the code should do) and the implementation (what the code does). A test that passes is a vote from the implementation that it matches the specification. You have also seen feedback loops (T11) throughout this book: the test suite is the feedback loop that makes regressions visible before they reach production. The speed of the feedback loop determines its effectiveness.