The roots of folk biology.

As adults, we think about the biological world in different ways than other realms. We are, for example, more inclined to see purposes of just the parts of animals and plants and not of the plant or animal as a whole, but we happily do so for artifacts (1). We see living kinds as occupying relatively unique locations in taxonomies (2). We make a much richer set of inferences about shared properties for all living kinds as opposed to all artifacts, we assume systematic patterns of growth and change not found elsewhere, and we often mistakenly assume there is a vitalistic force underlying all living things (3). Finally, we infer an unseen essence that is responsible for surface properties (4). In a casual walk through any environment, urban or rural, we typically are highly confident about whether we are encountering living things. Although the recognition of a distinct world of living thingsmight seem to be a gradual consequence of culture and schooling, in PNAS, Setoh et al. (5) suggest something quite remarkable—well before they have understood a single word, infants at least as young as 8mo have different expectations about a large part of the living world, namely animals. In particular, they expect animals to have filled interiors.