Dennett (1942–) engages evolutionary theory by asking the question, “Skyhooks or cranes?” Skyhooks are unexplained leaps from one stage of development to the next, whereas cranes are ways of understanding a later stage based on the design of an earlier one. Dennett has argued that consciousness, the contents of consciousness, and even the products of consciousness, such as Shakespeare’s plays, can be naturalistically understood in the same way that physical evolution is understandable. Neural systems create “multiple drafts” of the same thing so that the brain itself is “a sort of dung heap in which the larvae of other people’s ideas renew themselves.” Dennett is also a proponent of the doctrine of “memes,” whereby certain patterns of behavior are products of evolution that are physically inherited. His extreme materialism has attracted many critics, as well as supporters.