This book examines the nature of Wittgenstein's famous 1913 objection to Russell's theory of judgment, and its implications for subsequent developments in Russell's philosophy in the early decades of the twentieth century. The objection was a subtle point of logic, yet its significance for Russell led to his abandonment of a large project on theory of knowledge and to a substantial re-thinking of his philosophical logic, the result being his Philosophy of Logical Atomism lectures of 1918.