In 1982 Vision: A Computational Investigation into the Human Representation and Processing of Visual Information by the British neuroscientist David Marr, a professor at MIT, was published posthumously in New York. This work defined a general framework for studying complex biological systems.
"According to Marr, a complex biological system can be understood at three distinct levels. The first level ("computational level") describes the input and output to the system, which define the task the system is performing. In the case of the visual system, the input might be the image projected on our retina and the output might our brain's identification of the objects present in the image we had observed. The second level ("algorithmic level") describes the procedure by which an input is converted to an output, i.e. how the image on our retina can be processed to achieve the task described by the computational level. Finally, the third level ("implementation level") describes how our own biological hardware of cells implements the procedure described by the algorithmic level" (Yarden Katz, "Noam Chomsky on Where Artificial Intelligence Went Wrong," Atlantic Monthly, 11-1-2012).