This article presents a detailed review of economist Frank H. Knight's major arguments for capital as the only factor of production. Besides serving the purpose of those interested in Knight and the history of thought, this exposition provides a wider context in which to view the modern work on human capital. When he said that capital is the only factor of production, Knight meant simply that land and labor are in all essential respects identically like capital and consequently should be included under that common name. Instead of the standard three factors, economics should have a single factor of production called capital. Knight's defense of this proposition was not well organized. His basic technique was to throw up for consideration every possible ground on which one might be tempted to argue for distinguishing land and labor from capital and then try to show that none of them were valid. He also presented important positive reasons for including land and labor in capital, but the focus and emphasis was on the lack of grounds by which they could be distinguished.