To give a computational account of a physical system is to explain its outward behaviour by reference to certain functionally defined interpretable inner states and processes. The operation of the processor can itself be seen as a process, call it P ', internal to P , which is the result of an interior processor Q ' manipulating its field of symbolic structures. Within each process we can define an ingredient processor and its field of application until we reach a processor which simply performs the hardwired electronic functions of the machine. This is called serial reduction. It provides a means whereby complex procedures can be broken down into sequences of simpler tasks until we reach the ground-level operations of the machine.
Desires are different behaviour-causing computational relations to just such mental representations.
Psychological processes can be regarded as computational processes if they can be formally defined in terms of sequences of operations for manipulating interpretable symbols. The crucial remaining difference between ourselves and machines is that whereas we have to attribute semantic content to the machine's representations, or data structures, semantics arises in us without external attribution. Thus creatures with different neurophysiological states can share the same computational states and processes. So to establish a distinct computational level in a creature is to establish that it has states with certain semantically interpretable contents. B.C.S. See also Mind, syntax, and semantics .