Actually, this phrase is one of the bases of Western
logic (and I mean logic as a branch of
philosophy). It appeared first in 'De Logica', the
Latin translation of
Aristotle's 'Peri Logikes', or in
English: 'On Logic'. And indeed it refers only to logic. This phrase was never used by
Aristotle, or any other
philospher, in any other context, neither moral nor any other.
In logic tertium non datur means simply that any given statement can be either true or false, and there is no other option, since it cannot be neither true nor false, and it cannot be both true and false.
What, might one ask, about the maybes, sometimes somewheres et cetera? well, in this case, all one needs is to expand his statement in order to include those maybes, sometimes, somewheres et cetera, and again the statement will be presented as either truth or fallacy, and a third (option) is not given.