## Implications of Universality

When we first discussed cellular automata, Turing machines, substitution systems, register machines and so on in Chapter 3, each of these kinds of systems seemed rather different. But already in Chapter 3 we discovered that at the level of overall behavior, all of them had certain features in common. And now, finally, by thinking in terms of computation, we can begin to see why this might be the case.

The main point, as the previous two sections [5, 6] have demonstrated, is that essentially all of these various kinds of systems—despite their great differences in underlying structure—can ultimately be made to emulate each other.

This is a very remarkable result, and one which will turn out to be crucial to the new kind of science that I develop in this book.

In a sense its most important consequence is that it implies that from a computational point of view a very wide variety of systems, with very different underlying structures, are at some level fundamentally equivalent. For one might have thought that every different kind of system that we discussed for example in Chapter 3 would be able to perform completely different kinds of computations.

But what we have discovered here is that this is not the case. And instead it has turned out that essentially every single one of these systems is ultimately capable of exactly the same kinds of computations.

And among other things, this means that it really does make sense to discuss the notion of computation in purely abstract terms, without referring to any specific type of system. For we now know that it ultimately does not matter what kind of system we use: in the end essentially any kind of system can be programmed to perform the same computations. And so if we study computation at an abstract level, we can expect that the results we get will apply to a very wide range of actual systems.

But it should be emphasized that among systems of any particular type—say cellular automata—not all possible underlying rules are capable of supporting the same kinds of computations.

Indeed, as we saw at the beginning of this chapter, some cellular automata can perform only very simple computations, always yielding