Religion is one of the most universal and most studied human phenomena, yet there exists no widely shared definition of religion. This ambitious study attempts to provide and defend such a definition. Stewart Guthrie argues that religion is best understood as systematic anthropomorphism: the attribution of human characteristics to non-human things or events. Many writers have seen anthropomorphism as a superficial characteristic of religion. For Guthrie, however, it is central; religion consists in seeing the world as human-like.

Guthrie begins by demonstrating that we find plausible, in varying degrees, a continuum of human-like beings including gods, spirits, demons, gremlins, abominable snowmen, Hal the Computer, and Chiquita Banana. We find messages from such beings in phenomena such as weather, earthquakes, plagues, traffic accidents, and the flight of birds. Guthrie argues that this represents an adaptive strategy; we "bet" on the most important possible interpretation of our perceptions of our world - it is better to mistake a boulder for a bear than the other way around. Because of the extreme importance for us of other human beings and their actions, we project human characteristics onto what we see. Guthrie then shows how this explanation can be applied to virtually every belief and experience classified as religious. The result is a provocative and disturbing book that should be both influential and controversial.