whether, say, a particular pattern would ever die out in the evolution of a given cellular automaton. But from the discussion of the previous section we know that this in general is undecidable.

So it follows that it must be undecidable whether a given integer equation of some particular general form has a solution. And from the arguments above this in turn implies that there must be specific integer equations that have no solutions but where this fact cannot be proved from the normal axioms of arithmetic.

So how ultimately can this happen?

At some level it is a consequence of the involvement of infinity. For at least in a universal system like arithmetic any question that is entirely finite can in the end always be answered by a finite procedure.

But what about questions that somehow ask, say, about infinite numbers of possible integers? To have a finite way to address questions like these is often in the end the main justification for setting up typical mathematical axiom systems in the first place.

For the point is that instead of handling objects like integers directly, axiom systems can just give abstract rules for manipulating statements about them. And within such statements one can refer, say, to infinite sets of integers just by a symbol like s.

And particularly over the past century there have been many successes in mathematics that can be attributed to this basic kind of approach. But the remarkable fact that follows from Gödel's Theorem is that whatever one does there will always be cases where the approach must ultimately fail. And it turns out that the reason for this is essentially the phenomenon of computational irreducibility.

For while simple infinite quantities like 1/0 or the total number of integers can readily be summarized in finite ways—often just by using symbols like ∞ and Aleph_{0}—the same is not in general true of all infinite processes. And in particular if an infinite process is computationally irreducible then there cannot in general be any useful finite summary of what it does—since the existence of such a summary would imply computational reducibility.