My three visions of computers in education, as described above, could be considered as based, respectively, on those three outside-in metaphors - computer-as-source, computer-as-tutor, and computer-as-prosthetic. However, the third vision has a twist which enables us to escape the outside-in framework. It is based on the metaphor of computer as-positive-prosthetic. It does not supplant some biological system which is defective but supplements a biological system which is working perfectly well. This metaphor implies a synergy between natural and artificial intelligence. It is the metaphor used, not in the familiar artificial intelligence (AI) tradition in computer science, but in the intelligence amplification (IA) tradition [Rheingold 1985].

      Acquiring the operating manual is largely a matter of learning how to use tools (the word acquiring is judiciously chose to stress the fact that it is an inside-out job). As the wonderful genetic potential - given to all members of our species as a conception-day gift - unfolds, we reach out for tools which help us realise this human potential. Education could be considered as the process of augmenting the information in the zygote, which represents the experience of our species, with the information we each acquire in our individual lifetimes. There are extragenetic tools (outside the genetic code) and there are extrasomatic tools (outside the body) [Sagan 1977]. The computer is the latest, and most dramatic, extrasomatic tool. It extends the function of the brain as the telescope extends the function of the eye and the car extends the function of the leg. It is a powerful positive prosthetic. To refuse to use this tool is to try to outsee the telescope and to outrun the car.