In the last post we saw that in order for a type to be considered real – that is, a natural object – a group of individuals must have more in common with one another than with any individual excluded from that group. It’s already quite clear that by this definition, Simon Baron-Cohen’s “brain types” are not really types at all. Look at the graph again, and you’ll see that many individuals actually have more in common (more similar scores) with another individual on the other side of a boundary line than with most individuals within their own colour zone. .

Those stripes of colour don’t even represent distinct types of score, let alone distinct types of brain. They are nothing more than stripes of colour painted across an arrangement of individuals’ results on two self-reporting questionnaires.¹

And that isn’t all:

Even if Baron-Cohen’s vague and arbitrary groupings did tick all the boxes to be considered “types” in the taxonomic sense, typological thinking actually went out of fashion ages ago as evolutionary biology gained currency. There are those, like Wilkins and Ebach (quoted in my last post) who argue that typology does have a place in biology – in descriptions, at least – but as they themselves point out, typology is now generally “regarded as a regressive and pre-evolutionary approach to the data and biology.”² The question, then, is whether types exist at all.

Let’s go back to the beginning.

The Linnaean taxonomy is so much a part of our tradition that most of us will have grown up feeling that the grouping and naming of living things by genus and species – as in homo sapiens and my personal favourite, rattus rattus – reflect a tangible genetic reality that exists outside of human thought.

But “species” and “genus” were concepts that Carl Linnaeus inherited from Plato and Aristotle. It was in an ancient tradition, and in perfect freedom from the framework of evolutionary theory and genetics, that in 1735 Linnaeus constructed the system of classification we still use today. Yes, it has evolved with evolutionary theory; indeed, Linnaeus modified it a number of times himself. Nevertheless, its whole aim, as well as its structure and concepts, is rooted in a tradition of essentialism.

Very, very briefly: Plato, like many philosophers before him, observed that the world around him was in constant flux: unstable and always changing. He proposed that behind this wobbly material world there was another, more stable reality: the world of ideas. Material world objects like Plato himself, like a particular horse or a particular house, were in fact only imperfect shadows of the perfect and unchanging ideas or forms, man, horse and house. These ideal forms were called species. Plato is of the species man. Species were grouped into genera (plural of genus). Man and horse are both of the genus animal; house may be said to be of the genus building.

Next came Aristotle, who termed the defining idea or form of a thing its τὸ τί ἦν εἶναι, conventionally rendered in English as essence; the most common literal translation is “what-it-is-to-be.” There are other translations, but they generally communicate this sense of design: of what something is supposed to be.

And there you have the basis of essentialism. It’s a doctrine that has enjoyed profound – perhaps unparalleled – influence. Medieval Christian churchmen thought in these terms: God had created an natural order of unchanging forms, of which the material forms were imperfect representations. Early naturalists like Linnaeus were working within this tradition of Christian and essentialist thought. The members of a species shared a unique and unchanging essence, and the essence defined the species.

Trouble was, it actually proved rather tricky to locate the darned thing. To find a trait that is shared by every single member of every generation of a species for its entire life on the planet, and not shared by any member of any other species, is nigh on impossible.

R.I.P. Essentialism; enter Darwinism.

In theory, that is. In practice, the death of essentialism is about as much a natural object as were those elusive essences themselves.