Picture an economy stuck in a deep slump. Factories stand idle, millions are out of work, and the usual advice — wait, let markets adjust — feels like telling a drowning person to trust the tide. In the 1930s, John Maynard Keynes looked at that suffering and argued that the patient needed more than rest; it needed deliberate medicine. This chapter explores how Keynes upended the economics of his day, why he insisted we cannot rely on automatic fixes, and how his ideas still shape the way we think about recessions, government action, and the messy human emotions that drive prosperity and panic.