The Individuality Thesis

Language is full of terms that we use both in a concrete and in an abstract sense. With the term chair, for example, we can refer to the 

We also have words that can identify groups of objects, for example armies and species, and in these cases we normally employ different terms for different types of groups and subgroups. Armies, for example, are made of platoons, companies, regiments and divisions. Animals are classified into taxa such as species, genera, families, and so on. We distinguish, in other words, between individuals and groups of individuals , or classes , where classes are abstract entities while individuals are concrete bodies. 

A species is a collection of individuals, i.e. a class, but it is also a concept that seems to have been devised to drive us mad. If we accept the nominalist position (that only individuals are real), we must conclude that species, being classes, are abstract groups, not real things, and if they are not real they cannot evolve. This is enough to make us embrace the realist position, and say that species are real, but this turns into another headache, because now we have to explain how on earth can an abstract thing like a class be abstract and real at the same time. One can almost hear the nominalists grin It s either abstract or real, you cannot have it both ways.  

At this point we keep calm, take a deep breath, and sweep 

This is Ghiselin s Individuality Thesis (also known as the Radical Solution), and one could be forgiven for thinking that there must be a trick, somewhere. Except that there is no trick, because Ghiselin has not just said that species are individuals. He has proved it. 

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