undecidability—this is no longer the case, and as I discussed above I suspect that it will actually be quite common for there to be all sorts of short theorems that have only extremely long proofs.

No doubt many such theorems are much too difficult ever to prove in practice. But even if they could be proved, would they be considered interesting? Certainly they would provide what is in essence new information, but my strong suspicion is that in mathematics as it is currently practiced they would only rarely be considered interesting.

And most often the stated reason for this would be that they do not seem to fit into any general framework of mathematical results, but instead just seem like isolated random mathematical facts.

In doing mathematics, it is common to use terms like difficult, powerful, surprising and deep to describe theorems. But what do these really mean? As I mentioned above, any field of mathematics can at some level be viewed as a giant network of statements in which the connections correspond to theorems. And my suspicion is that our intuitive characterizations of theorems are in effect just reflections of our perception of various features of the structure of this network.

And indeed I suspect that by looking at issues such as how easy a given theorem makes it to get from one part of a network to another it will be possible to formalize many intuitive notions about the practice of mathematics—much as earlier in this book we were able to formalize notions of everyday experience such as complexity and randomness.

Different fields of mathematics may well have networks with characteristically different features. And so, for example, what are usually viewed as more successful areas of pure mathematics may have more compact networks, while areas that seem to involve all sorts of isolated facts—like elementary number theory or theory of specific cellular automata—may have sparser networks with more tendrils.

And such differences will be reflected in proofs that can be given. For example, in a sparser network the proof of a particular theorem may not contain many pieces that can be used in proving other theorems. But in a more compact network there may be intermediate definitions and concepts that can be used in a whole range of different theorems.