“Not a day passes without its discipline, whether for good or for evil.”
Daily experience is always training something, so ordinary choices can reinforce either constructive habits or patterns that make later self-command harder.
Source details
Character, Chapter I, “Influence of Character,” Project Gutenberg #2541. Smiles introduces the sentence while explaining that character is formed by many minute circumstances under partial individual control, so each day contributes discipline for good or ill.