Tertium non datur – The reconciling “third,” not logically foreseeable, characteristic of a resolution in a conflict situation when the tension between opposites has been held in consciousness. (See also transcendent function.)
As a rule it occurs when the analysis has constellated the opposites so powerfully that a union or synthesis of the personality becomes an imperative necessity. … [This situation] requires a real solution and necessitates a third thing in which the opposites can unite. Here the logic of the intellect usually fails, for in a logical antithesis there is no third. The “solvent” can only be of an irrational nature. In nature the resolution of opposites is always an energic process: she acts symbolically in the truest sense of the word, doing something that expresses both sides, just as a waterfall visibly mediates between above and below. [“The Conjunction,” CW 14, par. 705.]
© from Daryl Sharp’s Jung Lexicon, reproduced with kind permission of the author.