Subcontrary

Part of speech: adjective, noun

Definitions

1. In logic, describing two propositions that are opposed in such a way that they cannot both be false at the same time, though both may be true
2. Denoting a logical relationship between two statements where at least one must be true, even though they might both hold simultaneously
3. Relating to propositions in logic that are mutually opposed in quality or quantity but share the property that both cannot fail to be true at once

Etymology: "Subcontrary" is a plainly built word: the prefix "sub-", meaning "under" or "below" in Latin, joined to "contrary," which comes from Latin "contrarius," literally "opposite" or "against." The compound is essentially a description of position, one thing placed beneath another in the opposing direction. The term's real life is in logic and geometry rather than everyday speech. In traditional Aristotelian logic, a "subcontrary" pair describes two propositions, such as "some S is P" and "some S is not P," which can both be true but cannot both be false. The prefix signals that they sit below the contraries in the familiar square of opposition, where the opposing statements are placed on a diagram.