Human reasoning is characterized by psychological essentialism. In reasoning about objects we distinguish between deep essential properties that define the objects’ identity and superficial properties that can be changed without altering the identity of the object. Painting a tiger like a crocodile, e.g., does not turn it into a crocodile.

Essentialist reasoning has been amply documented in adults and older children from age four (Gelman, 2003; Keil, 1982). Little is known so far about the roots of psychological essentialism, both ontogenetically and phylogenetically. In particular, it is unclear whether psychological essentialism is based on the acquisition of linguistic means (such as kind terms) and is therefore uniquely human, or whether it is a more fundamental cognitive capacity possible without language. In a series of experiments with human infants and non-human apes we explored whether sortal object individuation in these subjects already involves essentialist modes of thinking.