Tertium comparationis

Tertium comparationis (Latin = the third [part] of the comparison[?]) is the quality that two things which are being compared have in common. It is the point of comparison which prompted the author of the comparison in question to liken someone or something to someone or something else in the first place.

If a comparison visualizes an action, state, quality, object, or a person by means of a parallel which is drawn to a different entity, the two things which are being compared must necessarily not be identical. However, they must possess at least one quality in common. This common quality has traditionally been referred to as tertium comparationis.

The most common devices used to achieve this are metaphors and similes, especially, but by no means exclusively, in poetic language. As in many cases one aspect of the comparison is implied rather than made explicit, the hidden part will surface if the comparison is presented in the form of a mathematical ratio.

Examples

• Necessity is the mother of invention. (English proverb)

mother : daughter :: necessity : invention

Tertium comparationis: source, where something derives from

• Woman is the nigger of the world. (John Lennon)

nigger : USA (historically) :: woman : world

Tertium comparationis: bad, inhumane treatment, discrimination

• Goodbye, England's rose. (Elton John on the death of Princess Diana)

This is a classical metaphor.

Tertium comparationis: beauty

• Fighting for peace is like fucking for virginity.

Tertium comparationis: the futility of the effort

If they [our two souls] be two, they are two so

As stiff twin compasses are two;

Thy soul, the fixed foot, makes no show

To move, but doth, if th'other do.

(John Donne: "A Valediction: Forbidding Mourning") (Read the whole poem (http://eir.library.utoronto.ca/rpo/display/poem682).)

Tertium comparationis: the link between the two allegedly separate parts which will always be there so that one can never be severed from the other