discussed in Chapter 9 it would actually be possible to establish this with complete certainty. For such a theory would have the feature that it could be emulated by a universal system of the type I discussed in the previous chapter—with the result that nowhere in our universe could computations ever occur that are more sophisticated than those carried out by the universal systems we have discussed.

So what about computations that we perform abstractly with computers or in our brains? Can these perhaps be more sophisticated? Presumably they cannot, at least if we want actual results, and not just generalities. For if a computation is to be carried out explicitly, then it must ultimately be implemented as a physical process, and must therefore be subject to the same limitations as any such process.

But as I discussed in the previous section, beyond asserting that there is an upper limit to computational sophistication, the Principle of Computational Equivalence also makes the much stronger statement that almost all processes except those that are obviously simple actually achieve this limit.

And this is related to what I believe is a very fundamental abstract fact: that among all possible systems with behavior that is not obviously simple an overwhelming fraction are universal.

So what would be involved in establishing this fact?

One could imagine doing much as I did early in this book and successively looking at every possible rule for some type of system like a cellular automaton. And if one did this what one would find is that many of the rules exhibit obviously simple repetitive or nested behavior. But as I discovered early in this book, many also do not, and instead exhibit behavior that is often vastly more complex.

And what the Principle of Computational Equivalence then asserts is that the vast majority of such rules will be universal.

If one starts from scratch then it is not particularly difficult to construct rules—though usually fairly complicated ones—that one knows are universal. And from the result in the previous chapter that rule 110 is universal it follows for example that any rule containing this one must also be universal. But if one is just given an arbitrary rule—