Tertium quid

Electrum, a rare alloy of gold and silver, was used by Tertullian as an example of a tertium quid, by which he meant a mixture with composite properties.

Tertium quid refers to an unidentified third element that is in combination with two known ones. The phrase is associated with alchemy. It is Latin for "third thing", a translation of the Greek tríton ti (τρίτον τι). The Greek phrase was used by Plato (360 BC), and by Irenæus (c. AD 196). The earliest Latin example is by Tertullian (c. 220), who used the phrase to describe a mixed substance with composite properties such as electrum, a somewhat different sense than the modern meaning.

In Christology

In the Christological debates of the fourth century, it was used to refer to the followers of Apollinaris who spoke of Christ as something neither human nor divine, but a mixture of the two, and therefore a "third thing".

In American political history

In American political history, the Tertium Quids, or Quids, were moderate members of Jefferson's Democratic-Republican Party. The word implies that their political position was apt to embrace true republicanism and the comparable conservatism of the Federalist Party, particularly on foreign policy. The Quids arose in 1804 during Thomas Jefferson's first term in office. They were led by Virginia's John Randolph of Roanoke. They stood by the party's original stance for strict construction of the Constitution and opposed Jefferson's pragmatic approach to governing.

In sociology

In sociology, it describes a category of degraded moral consideration.

In Souls of Black Folk, W.E.B. Du Bois used the term "tertium quid" to refer to the identity of African Americans in a racist society, where non-white people are viewed as a devalued category between man and animal.

"The second thought streaming from the death (slave)-ship and the curving river is the thought of the older South, the sincere and passionate belief that somewhere between men and cattle, God created a tertium quid, and called it a Negro—a clownish, simple creature, at times even lovable within its limitations, but straitly foreordained to walk within the Veil."

In American law

The term is used in the important Supreme Court case Wal-Mart Stores, Inc. v. Samara Brothers, Inc. 529 U.S. 205 (2000). In this Lanham Act case, the Court, when discussing product packaging vs. product design, referred to the type of trade dress in its earlier Two Pesos decision as "some tertium quid" that may be a mutation of product packaging and product design—a "third thing."

In literature

In literature, it can describe an adulterer, often in a cuckolded relationship.

Kipling employs the term in "At the Pit’s Mouth," for an adulterer: "Once upon a time there was a Man and his Wife and a Tertium Quid."

Talbot Mundy a contemporary of Kipling makes use of the term in "King of the Khyber Rifles" (p. 265) to describe a cuckold, "And what kind of man must Rewa Gunga be who could lightly let go all the prejudices of the East and submit to what only the West has endured hitherto with any complacency--a "tertium quid"? "

Also, Robert Browning uses the term "Tertium Quid" in his long narrative poem "The Ring and the Book" to describe a speaker with a third point of view who has a different, more balanced, opinion on the 1698 Roman murder case his poem discusses, different from the opinions of "Half Rome" and "The Other Half Rome" who strongly sympathize with, or strongly do not sympathize with, the accused.