counterinstance also called counterexample. (1) A particular instance of an argument form that has all true premises but a false conclusion, thereby showing that the form is not universally valid. The argument form ‘p 7 q, – p / , ~q’, for example, is shown to be invalid by the counterinstance ‘Grass is either red or green; Grass is not red; Therefore, grass is not green’. (2) A particular false instance of a statement form, which demonstrates that the form is not a logical truth. A counterinstance to the form ‘(p 7 q) / p’, for example, would be the statement ‘If grass is either red or green, then grass is red’. (3) A particular example that demonstrates that a universal generalization is false. The universal statement ‘All large cities in the United States are east of the Mississippi’ is shown to be false by the counterinstance of San Francisco, which is a large city in the United States that is not east of the Mississippi. V.K. counterpart theory, a theory that analyzes statements about what is possible and impossible for individuals (statements of de re modality) in terms of what holds of counterparts of those individuals in other possible worlds, a thing’s counterparts being individuals that resemble it without being identical with it. (The name ‘counterpart theory’ was coined by David Lewis, the theory’s principal exponent.) Whereas some theories analyze ‘Mrs. Simpson might have been queen of England’ as ‘In some possible world, Mrs. Simpson is queen of England’, counterpart theory analyzes it as ‘In some possible world, a counterpart of Mrs. Simpson is queen of (a counterpart of) England’. The chief motivation for counterpart theory is a combination of two views: (a) de re modality should be given a possible worlds analysis, and (b) each actual individual exists only in the actual world, and hence cannot exist with different properties in other possible worlds. Counterpart theory provides an analysis that allows ‘Mrs. Simpson might have been queen’ to be true compatibly with (a) and (b). For Mrs. Simpson’s counterparts in other possible worlds, in those worlds where she herself does not exist, may have regal properties that the actual Mrs. Simpson lacks. Counterpart theory is perhaps prefigured in Leibniz’s theory of possibility. See also COUNTERFACTUALS , POS- SIBLE WORLD. P.Mac.