Do ducks lay eggs?

Aristotle - De Anima (On the Soul)

Ducks lay eggs. True or false? Most people regard the statement as unquestionably true.

Sentences like this are called generics and the interesting feature of generics is that they tolerate exceptions.[1] For example, male ducks don't lay eggs and neither do female ducks that lack the capability due to defects. Yet most of us accept that the statement conveys real information about ducks as a species, despite such exceptions in individual ducks.

Generic sentences are problematic on a nominalist view since universal terms (such as "duck") are considered to be nothing more than a collective name for individuals.[2] So the sentence "ducks lay eggs" would seem to imply a hidden "all" quantifier and thus really means "all ducks lay eggs". So on a nominalist reading "ducks lay eggs" is considered either strictly false or perhaps not a legitimate knowledge claim at all. Whereas realists about universals hold that generic statements differ in meaning from statements that include quantifiers. That is, generic statements refer to universal categories or kinds, not to the individual members of those categories or kinds.

Greg N. Carlson explains the key feature of generics as follows:

Finally, to re-emphasize a point made by Goodman (1955) and more recently by Dahl (1975) (among a host of others), the truth of generics depends on a notion of non-accidental generalization for their truth. The world contains in its extension all manner of possible patterns and convergences, many of which we judge to be purely accidental, but others of which we take to be principled. Only the principled patterns are taken to support true generics.

Truth-Conditions of Generic Sentences: Two Contrasting Views

In Aristotelian terms, this generic meaning is captured by the distinction between essence and accident. One familiar example is the Aristotelian definition of humans as the rational animal. This definition successfully distinguishes between humans as a species and other animal species that lack that rational capability.[3] This is so even though individual exceptions exist. For example, a two-week old embryo, a comatose or sleeping human, or a human acting irrationally.

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[1] For a brief overview, see Generic Sentences and Predication.

[2] See my previous post on the problem of universals.

[3] For an interesting analysis of this definition, see Essentially Rational Animals.