computer technology to make single pictures of cellular automata, it requires considerably more to do large-scale systematic experiments.

Indeed, many of my discoveries about cellular automata came as direct consequences of using progressively better computer technology.

As one example, I discovered the classification scheme for cellular automata with random initial conditions described at the beginning of Chapter 6 when I first looked at large numbers of different cellular automata together on high-resolution graphics displays. Similarly, I discovered the randomness of rule 30 (page 27) when I was in the process of setting up large simulations for an early parallel-processing computer. And in more recent years, I have discovered a vast range of new phenomena as a result of easily being able to set up large numbers of computer experiments in Mathematica.

Undoubtedly, therefore, one of the main reasons that the discoveries I describe in this chapter were not made before the 1980s is just that computer technology did not yet exist powerful enough to do the kinds of exploratory experiments that were needed.

But beyond the practicalities of carrying out such experiments, it was also necessary to have the idea that the experiments might be worth doing in the first place. And here again computer technology played a crucial role. For it was from practical experience in using computers that I developed much of the necessary intuition.

As a simple example, one might have imagined that systems like cellular automata, being made up of discrete cells, would never be able to reproduce realistic natural shapes. But knowing about computer displays it is clear that this is not the case. For a computer display, like a cellular automaton, consists of a regular array of discrete cells or pixels. Yet practical experience shows that such displays can produce quite realistic images, even with fairly small numbers of pixels.

And as a more significant example, one might have imagined that the simple structure of cellular automaton programs would make it straightforward to foresee their behavior. But from experience in practical computing one knows that it is usually very difficult to foresee what even a simple program will do. Indeed, that is exactly why bugs in programs are so common. For if one could just look at a program