The authors have provided a plausible account of a cognitive bias operating early in development that forms an inchoate component of psychological essentialism. I would like to draw attention to two issues that I believe are pertinent to the article by Cimpian & Salomon (C&S). The first is that essentialism can actually be conceptualized by two apparently mutually exclusive principles that are related to identity. The first is the more familiar essentialist principle of quiddity, from the Latin quidditas, which translates as the “whatness” of some thing – in other words, the properties a thing shares with the other members of the category to which it belongs. Quiddity is most similar to the notion of Plato's form, where members of a category depart from some idealized version. In the target article, it is the essentialist principle of quiddity that is under consideration, because in order to infer an inherent feature to explain an observed pattern, one must be dealing with multiple members that constitute the group.

In contrast to quiddity, however, is the other essentialist notion of haecceity, from the Latin haecceitas, which translates as “thisness.” Aristotle, critiquing his mentor Plato's notion of form, drew attention to the problem of reconciling the notion of unique identity with category membership. Medieval Scottish philosopher John Duns Scotus (from whose name we get the term dunce) introduced and championed the term haecceity. A good analogy from biology that helps to draw the distinction between these two essentialist principles would be the genome that determines which species membership category an animal belongs to (cat, dog, mouse, etc.) as the quiddity, and the DNA that is unique to the individual animal (Felix, Fido, or Mickey) as the haecceity.

These are not only distinctions in essentialism but they also manifest in naïve intuitions about nature of the world. As C&S have pointed out, essentialism is at the core of the understanding the mechanisms of inheritance, which explains why it makes evolution by natural selection so counter-intuitive. As biologist Ernst Mayr (Reference Mayr1963) noted, if one intuits that the different species are essentially different, an identity that is passed on by inheritance, then it is problematic to accept that all life forms have a common original ancestor. Richard Dawkins (Reference Dawkins2010) calls this intuitive biological essentialism “the dead hand of Plato.”

C&S have argued that artefacts do not exhibit the hallmarks of psychological essentialism, but this is not necessarily true if one considers haecceity. In the classic account attributed to Roman writer Plutarch, the ship of Theseus is a thought experiment that evokes haecceity. In the story, the Greek king's ship is replaced plank by plank over time so that eventually it contains none of the original material. Plutarch asked rhetorically whether the ship retained the same identity and, moreover, if one was to reassemble all the pieces that had been removed into a second ship, which would be the ship of Theseus? The results of experimental studies with children and adults confirm that most individuals infer that individuals that are gradually replaced by component parts retain their identity (Hall Reference Hall1998). This is because we infer an essential element in addition to the material composition when we are asked to consider the unique nature of things. Hall's (Reference Hall1998) developmental study revealed a stronger essentialist perspective for living things compared to an artefact, but we will essentialize objects that we consider significant by virtue of their unique identity if they have sentimental value (Hood & Bloom Reference Hood and Bloom2008). I have conjectured that this holds especially true for emotional objects such as memorabilia associated with individuals that we revere, from celebrities to religious saints, as well as so-called murderbilia, which are the items associated with murderers (Hood Reference Hood2009). The authenticity of an object is similarly conferred by essentialist beliefs so much so that the value we place on objects such as artworks or collectibles is shaped by what we believe the object to be (Bloom Reference Bloom2010). For many, a perfect forgery indistinguishable from an original lacks some property that is difficult to articulate, consistent with the placeholder function that essentialism provides (Medin & Ortony Reference Medin, Ortony, Vosniadou and Ortony1989).