Indeed, abstract conceptual thought is held to be so central to being human that the idea of someone being incapable of this kind of thinking is a subject for (sometimes rather cruel) humor. Interest in understanding the capacity for abstract thought has been a matter of serious consideration that dates back at least three centuries to the famous English philosopher John Locke. Locke confidently contended that "brutes abstract not" (1) and insisted that exhibiting abstract thought definitively divided humans from all other animals. However, no science then existed to confirm or refute Locke's contention. On page 286 of this issue, Martinho and Kacelnik (2) put the claim that animals are incapable of abstract thought to a strong behavioral test.