The dictum is usually phrased as, "make illegal states unrepresentable". Perhaps this is just playing with semantics, but, to me, that difference in wording is

The dictum is usually phrased as, "make illegal states unrepresentable". Perhaps this is just playing with semantics, but, to me, that difference in wording is incredibly important. An unauthorized state (which I'm interpreting here to mean an error state) may not be on the happy path, but that doesn't necessarily mean it's invalid. For example, "the user is trying to open a file, but they don't have permission to read it" is a valid state. The resulting error condition is also valid, even if it's not somewhere they want to find themself. An illegal state is, in the context of this phrase, meant to be one that's nonsensical. Perhaps "invalid" would be a better word. Think negative array indexes in languages that only let you index from the front of the array, or objects that are only halfway initialized. Situations where, having entered such a state, the behavior of the program is now undefined. We should strive to make it physically impossible get the program into a state where its behavior is undefined.