  The symbolic expressions over which cognitive processes operate, as well as the rules according to which these operations proceed, appear as representational states internal to the cognizing agent. They are individuated in terms of what they are about (phonemes, light intensity, edges, shapes, etc.). All of this cognitive activity takes place in the agent’s nervous system. It is in virtue of the activation of the nervous system that stimuli become encoded into a “mentalese” language of thought, akin to the programming languages found in ordinary computers; similarly, the rules dictating the manipulation of symbols in the language of thought are like the instructions that a C.P.U. executes in the course of carrying out a task. Rather than running spread sheets or displaying Tetris pieces, the computational brain produces language, or perceives the world, or retrieves items from memory. The methods of computational cognitive science reflect these ontological commitments. Experiments are designed to reveal the content of representational states or to uncover the steps by which mental algorithms transform input into output.