The following is the handout that accompanied a paper presented at the December 2001 Messina conference on Horizons in Complex Systems. Currently it presents the basic concepts only. It proposes that one key idea constitutes the key to understanding the brain, namely the fact that abstractions are possible. The particular abstractions relevant to particular aspects of nature define the designs that are capable of handling these aspects of nature. The task of explaining the brain thus reduces to an investigation of the abstractions, the relevant interrelationships, and the corresponding design components.