Pigeons can separate highly varied pictures of natural scenes and objects into two categories, after learning that responding to one category produces food. The criteria are based on the pictured presence of absence of water or of a particular person. In similar experiments, leaves of one species of tree were distinguished from those of other trees. This ability suggests that pigeons entertain simple concepts such as “food is available where water can be seen, or where a particular woman is visible, or where the leaves are those of white oaks, but not other trees.”