behavior. And in fact, this is precisely why it is conceivable that a simple program could reproduce all the complexity we see in physics.

Given a particular underlying program, it is always in principle possible to work out what it will do just by running it. But for the whole universe, doing this kind of explicit simulation is almost by definition out of the question. So how then can one even expect to tell whether a particular program is a correct model for the universe? Small-scale simulation will certainly be possible. And I expect that by combining this with a certain amount of perhaps fairly sophisticated mathematical and logical deduction, it will be possible to get at least as far as reproducing the known laws of physics—and thus of determining whether a particular model has the potential to be correct.

So if there is indeed a definite ultimate model for the universe, how might one set about finding it? For those familiar with existing science, there is at first a tremendous tendency to try to work backwards from the known laws of physics, and in essence to try to "engineer" a universe that will have particular features that we observe.

But if there is in fact an ultimate model that is quite simple, then from what we have seen in this book, I strongly believe that such an approach will never realistically be successful. For human thinking—even supplemented by the most sophisticated ideas of current mathematics and logic—is far from being able to do what is needed.

Imagine for example trying to work backwards from a knowledge of the overall features of the picture on the facing page to construct a rule that would reproduce it. With great effort one might perhaps come up with some immensely complex rule that would work in most cases. But there is no serious possibility that starting from overall features one would ever arrive at the extremely simple rule that was actually used.

It is already difficult enough to work out from an underlying rule what behavior it will produce. But to invert this in any systematic way is probably even in principle beyond what any realistic computation can do.

So how then could one ever expect to find the underlying rule in such a case? Almost always, it seems that the best strategy is a simple one: to come up with an appropriate general class of rules, and then just