“Your future stands before you like a block of unwrought marble. You can work it into what you will.”
Conwell uses the image of uncarved marble to stress deliberate agency: future outcomes are shaped in part by the choices and sustained effort a person brings to them.
Source details
What You Can Do With Your Will Power, Chapter I, Project Gutenberg #33952. Conwell compares the future to unwrought marble while arguing for will and determination; the surrounding 1917 passage makes sweeping claims about heredity, environment, health and success. This remains historical motivational rhetoric rather than modern psychological or scientific evidence.