[L., app. rendering Gr. τρίτον τι, ‘some third thing.’] Something (indefinite or left undefined) related in some way to two (definite or known) things, but distinct from both.
(Gr. τρίτον τι occurs in Plato, Sophist, 250. The Latin form is in Irenæus, Adv. Her., 2. 1. 3 (c. 196), where it doubtless represents τρίτον τι of the lost Greek original; also, in Tertullian, Adv. Praxean, 27 (a. 220), and tertium nescio quid in Hilary, Synod., 73 (c. 358). The passage in Tertullian mentions electrum as an example of a body produced by the mixture of gold and silver; and app. tertium quid was used by the alchemists of a third substance different from its two constituents: see quot. from Bailey, and cf. next. Examples of the phrase in English context are late.)
[1613. Theatrum Chemicum, Index, Tertium quid. 1101, 1085.
1724. Bailey, Tertium Quid, (among Chymnists) the Result of the Mixture of some two Things, which forms something very different from both. L[atin].
[1809–10. Coleridge, Friend (1818), I. 157. The baleful product or tertium Aliquid, of this union retarded the civilization of Europe for Centuries.]
1826. Edin. Rev., Sept., 255. Balancing the opinions of Gall against those of Spurzheim, or compounding out of them a tertium quid.
1881. R. Adamson, Fichte, v. 110 While … we appear to assert that the two orders of facts make up all that is, we have in reality placed alongside of them … the thinking subject or mind, a tertium quid which certainly stands in need of some explanation.
1902. Menzies, Demonic Possess. N. T., vi. 187. The achievement was either devilish or divine. There was no tertium quid.
