Another reservation has to do with the verb "will." No serious futurist deals in "predictions." These are left for television oracles and newspaper astrologers. No one even faintly familiar with the complexities of forecasting lays claim to absolute knowledge of tomorrow. In those deliciously ironic words purported to be a Chinese proverb: "To prophesy is extremely difficult – especially with respect to the future."

This means that every statement about the future ought, by rights, be accompanied by a string of qualifiers – ifs, ands, buts, and on the other hands. Yet to enter every appropriate qualification in a book of this kind would be to bury the reader under an avalanche of maybes. Rather than do this, I have taken the liberty of speaking firmly, without hesitation, trusting that the intelligent reader will understand the stylistic problem. The word "will" should always be read as though it were preceded by "probably" or "in my opinion." Similarly, all dates applied to future events need to be taken with a grain of judgment.

The inability to speak with precision and certainty about the future, however, is no excuse for silence. Where "hard data" are available, of course, they ought to be taken into account. But where they are lacking, the responsible writer – even the scientist – has both a right and an obligation to rely on other kinds of evidence, including impressionistic or anecdotal data and the opinions of well-informed people. I have done so throughout and offer no apology for it.

In dealing with the future, at least for the purpose at hand, it is more important to be imaginative and insightful than to be one hundred percent "right." Theories do not have to be "right" to be enormously useful. Even error has its uses. The maps of the world drawn by the medieval cartographers were so hopelessly inaccurate, so filled with factual error, that they elicit condescending smiles today when almost the entire surface of the earth has been charted. Yet the great explorers could never have discovered the New World without them. Nor could the better, more accurate maps of today been drawn until men, working with the limited evidence available to them, set down on paper their bold conceptions of worlds they had never seen.