This document discusses two approaches to abstracting cognitive systems: 1) Marr's model proposes three levels of abstraction - computational theory, representation and algorithms, and hardware implementation. The computational level is most important and should be developed independently of lower levels. 2) Scott Kelso's model proposes the three levels are tightly coupled and mutually dependent - the environment influences behavior, and the physical system can simplify computations. Properties of the physical system should not be abstracted away.