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.

--

[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.

The problem of universals

This apple is red and so is that stop sign. Bob has two arms and two legs.

The apple, the stop sign and Bob are all individuals or particular things. When we say that the apple is red or that Bob has two arms, we are predicating something of those individuals. And when the same thing can be predicated of more than one individual, then that thing (say, redness or the number two) is termed a universal. A universal is just a general characteristic that those individuals have in common.

Now the status of individuals seems straightforward. We say that there is an apple there - it exists and we can see it and also eat it. But the status of universals seems less obvious. Is redness something real? If so, how does it happen to be in many places at once, in both the apple and the stop sign? And also, the number two seems very abstract, not a physical kind of thing that can exist like the apple and Bob do.

In philosophy, this is called the problem of universals. Plato was the first to discuss the issue in depth and proposed that universals do exist, but they exist in a higher realm of the Forms. So there is an ideal or perfect form for redness, a form for the number two, and a form for every other universal. The (imperfect) individual things that we can see around us are said to participate in those perfect forms. This view is called Platonic realism.

Aristotle (who was Plato's student) strongly disagreed with Plato on this issue. In his view, universals were indeed real, but they were not separable from the individuals that they were found in. That is, there is no redness independent of particular things that are red. This view is called moderate or immanent realism - the universals are immanent within the individuals.

A third view, nominalism, denies that universals are real at all. There are only the names we use and there is no universal quality that they have in common.

My own view is that Aristotle's immanent realism best captures what we implicitly mean when we assert things about individuals. The characteristics that the individuals have in common are not merely naming conventions or the creations of humans. Instead they are necessary for the individuals to be observable in any form at all. Granting that, there also seems to be no good reason to posit a non-natural realm for universals to reside in. They are simply features or aspects of the natural world that we experience, albeit at a more abstract level that requires human intelligence to be able to identify and reflect on them.

The image above shows part of the School of Athens fresco by the Italian Renaissance artist Raphael. It depicts Plato pointing towards the heavens where he supposed the ideal forms to be. It also depicts Aristotle gesturing downwards to the here-and-now world that he held universals were a part of.