  I begin with a picture that is familiar to philosophers of psychology, Marr’s (1982, 24–5) distinction between three levels of explanation in cognitive science. The highest specifies the computational task accomplished by the system of interest. It tells us what the goal of the system is, specified in terms of what it computes.  The middle level describes the actual representations and algorithms that carry out the goal. The lowest level tells us how brain tissue or other material substrate, such as the parts of a machine, can implement the algorithm. Note that the levels of description are different perspectives on the same mental process - Marr thinks of vision as the construction of a 3D representation from a 2D input, and the three levels are three different ways to understand that construction. Other cognitive phenomena can also be understood in these three ways. 