Grote, George, Plato, and the other companions of Sokrates, Volume 2, p. 418. From the dialogue Sophist: "Existence or reality must therefore be a tertium quid, apart from motion and rest, not the sum total of those two items." (250b)
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Irenæus, Against Heresies 2.1.3. The surviving text is Latin, but the original would have been in Greek. "But if they
say this, there will be a 'tertium quid,' with this immense separation between the Pleroma and what is outside it, and this 'tertium quid' will limit and contain the other two, and will be greater than both the Pleroma and what is outside it, since it contains both in its bosom." (Grant, Robert McQueen, Irenaeus of Lyons, p. 108.)
