I am typing these words into a metaphorical document in a metaphorical window on a metaphorical desktop; the document is contained in a metaphorical file folder represented by a miniature picture of a real file folder. I know the desktop is metaphorical because it is vertical; if it were a real desktop, everything would slide to the bottom.

All this is familiar to anyone whose computer employs a graphical user interface (GUI). We use that collection of layered metaphors for the same reason we call unauthorized access to a computer a break-in and a machine language program burned into a computer chip, unreadable by the human eye, a writing. The metaphor lets us transport a bundle of concepts from one thing, about which that bundle first collected, to something else to which we think most of the bundle is appropriate. Metaphors reduce the difficulty of learning to think about new things. Well-chosen metaphors do it at a minimal cost in wrong conclusions.

Consider the metaphor that underlies modern biology: evolution as intent. Evolution is not a person and does not have a purpose. Your genes are not people either and also do not have purposes. Yet the logic of Darwinian evolution implies that each organism tends to have those characteristics that it would have if it had been designed for reproductive success.