Thinking about the Future

Bush's ideas of what the future holds for memory seem quantly divergent from our own present, as a few posts above have pointed out, but it raises the question how do people predict the future?

Bush comments breifly on this topic in section 8:
"In order that the picture may not be too commonplace, by reason of sticking to present-day patterns, it may be well to mention one such possibility, not to prophesy but merely to suggest, for prophecy based on extension of the known has substance, while prophecy founded on the unknown is only a doubly involved guess."

Bush's memex is interesting precisely because it is laid out in terms of a distinctly pre-digital age. How can someone on one side of such a revolution forsee the other side... or in this case even see the revolution out in the distance?
Neuromancer, for comparison, tossed aroun similarly far fetched ideas technologically but much of the ideas in neuromancer came to fruition within the same technological paradigm that it was writen - the paradigm of network computing. The memex was thrust from a paradigm of mechanical logic and analogue flims into a paradigm of digital data and computer manipulation that Bush simply could not forsee.