I've been looking for a clear expression of this idea for my netbooks in schools presentation. Because of the way computers are often used in schools (as instrumental tools to support a paper and ink derived curriculum) their potential revolutionary or transformational significance is often lost sight of. The virtue of this quote is that it clearly and concisely compares the difference between the computer medium and other media.
"Devices" which variously store, retrieve, or manipulate information in the form of messages embedded in a medium have been in existence for thousands of years. People use them to communicate ideas and feelings both to others and back to themselves. Although thinking goes on in one's head, external media serve to materialize thoughts and, through feedback, to augment the actual paths the thinking follows. Methods discovered in one medium provide metaphors which contribute new ways to think about notions in other media.Update (6th June 2009): There is a copy of the Kay / Goldberg paper, Personal Dynamic Media here
For most of recorded history, the interactions of humans with their media have been primarily nonconversational and passive in the sense that marks on paper, paint on walls, even "motion" pictures and television, do not change in response to the viewer's wishes. A mathematical formulation -- which may symbolize the essence of an entire universe -- once put down on paper, remains static and requires the reader to expand its possibilities.
Every message is, in one sense or another, a simulation of some idea. It may be representational or abstract. The essence of a medium is very much dependent on the way messages are embedded, changed, and viewed. Although digital computers were originally designed to do arithmetic computation, the ability to simulate the details of any descriptive model means that the computer, viewed as a medium itself, can be all other media if the embedding and viewing methods are sufficiently well provided. Moreover, this new "metamedium" is active -- it can respond to queries and experiments -- so that the messages may involve the learner in a two-way conversation. This property has never been available before except through the medium of an individual teacher. We think the implications are vast and compelling.