 The modification in sense brought about by the addition of the prefix is generally that of a simple negation: unworthy = “not worthy”, etc… The two terms [X, unX] are thus contradictory terms. But very often the prefix produces a “contrary” term or at any rate what approaches one: unjust generally implies the opposite of just; unwise means more than not wise and approaches foolish, unhappy is not far from miserable, etc.