Finally, the choice of what is considered scientifically interesting is certainly to a large extent arbitrary. Physics today uses enormous machines to investigate situations that exist for less than an enormously small amount of time, and we surely are entitled to employ all possible techniques to classify all experimentally observable phenomena. But we can at least ask one question: many phenomena of common experience, in themselves trivial (often to the point that they escape attention altogether!) - for example, the cracks in an old wall, the shape of a cloud, the path of a falling leaf, or the froth on a pint of beer - are very difficult to formalize, but is it not possible that a mathematical theory launched for such homely phenomena might, in the end, be more profitable for science?
The pre-Socratic flavor of the qualitative dynamics considered here will be quite obvious. If I have quoted the aphorisms of Heraclitus at the beginnings of some chapters, the reason is that nothing else could be better adapted to this type of study. In fact, all the basic intuitive ideas of morphogenesis can be found in Heraclitus: all that I have done is to place these in a geometric and dynamic framework that will make them some day accessible to quantitative analysis. The "solemn, unadorned words," like those of the sibyl that have sounded without faltering throughout the centuries, deserve this distant echo.