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A sentence which asserts its own falsity, e.g. "This sentence is false" or "I am lying". These paradoxical assertions are meaningless in the sense that there is nothing in the world which could serve to either support or refute them. Philosophers, of course, have a great deal more to say on the subject.
["The Liar: an Essay on Truth and Circularity", Jon Barwise and John Etchemendy, Oxford University Press (1987). ISBN 0-19-505944-1 (PBK), Library of Congress BC199.P2B37].