 The modification in sense brought about by the addition of the prefix [un-] is generally that of a simple negative: unworthy = ‘not worthy’, etc. The two terms [P, unP] are thus contradictory terms. But very often the prefix produces a “contrary” term…: unjust (and injustice) generally imply the opposite of just (justice); unwise means more than not wise and approaches foolish, unhappy is not far from miserable, etc.