Gordian knot, absurdity, ambiguity, ambivalence, antinomy, asymmetry, contradiction, crux, dilemma, disproportion, disproportionateness, enigma, equivocality, equivocation, heresy, heterodoxy, heterogeneity, hopelessness, impossibility, impossible, impossibleness, incoherence, incommensurability, incompatibility, inconceivability, incongruity, inconsistency, inconsonance, irony, irreconcilability, knot, knotty point, mystery, no chance, node, nodus, nonconformability, nonconformity, nonplus, oxymoron, perplexity, pons asinorum, poser, problem, puzzle, quandary, self-contradiction, teaser, the impossible, unconformability, unconformity, unimaginability, unorthodoxy, unthinkability, vexed question, what cannot be, what cannot happen