A part of the history of economic thought that has not previously been examined is the reception and resulting modification of Walras's theorem on the maximum utility of new capital goods. The reactions of his contemporaries and his responses to them, hidden for decades in his correspondence, reveal why and how he modified his theorem. His efforts to explain it to his correspondents and to persuade them of its correctness are invaluable as aids to understanding it because the presentation of the theorem in the Eléments is in some places difficult to follow and in some places incomprehensible. In this chapter, the criticisms of the theorem and Walras's responses are examined and evaluated, and a reformulation of the theorem is given.