A commitment held before the particular case arises. A principle costs something exactly when it is inconvenient.
Latin principium (beginning, foundation), from princeps (first, chief): primus (first) + capere (to take) — the one who takes first place; a principle is a founding commitment, held before any particular case tests it.
A commitment made before the hard case arrives, so that the hard case does not get to rewrite it. A principle that costs nothing when it is easy was never tested.