Do es mathematics need new axioms? Solomon Feferman The question, \Do es mathematics need new axioms?", is ambiguous in practically every resp ect. What do wemeanby `mathematics' ? What do wemeanby `need' ? What do wemeanby `axioms' ? You mighteven ask, What do wemeanby `do es' ? Part of the ambiguity lies in the various p oints of view from which this question might b e considered. The crudest di erence lies b etween the p oint of view of the working mathematician and that of the logician concerned with the foundations of mathematics. Nowsomeofmy fellow mathematical logicians might protest this distinction, since they consider themselves to b e just more of those \working mathematicians". Certainly, mo dern logic has established itself as a very resp ectable branch of mathematics, and there are quite a few highly technical journals in logic, suchasThe Journal of Sym- bolic Logic and the Annals of Pure and AppliedLogic, whose contents, from a cursory insp ection, lo ok just like those of other mathematical journals, setting sub jects aside. Lo oking even closer, you can pick upapaperon, say, the semi-lattice of degrees of unsolvability or the mo del theory of elds and not see it as any di erent in general character from a pap er on com- binatorial graph theory or cohomology of groups; they b elong to the same big frame of mind, so to sp eak. But if you pickupGodel's pap er on the incompleteness of axiom systems for mathematics, or his work and that of Cohen on the consistency and indep endence of the Axiom of Choice relative to the axioms of set theory,we're in a di erent frame of mind, b ecause we are doing what Hilb ert called metamathematics: the study of mathematics itself by the means of mathematical logic through its formalization in ax- iomatic systems. And it's that stance I want to distinguish from that of 1 the mathematician working on analysis or algebra or top ology or degrees of unsolvability, and so on. It's awkward to keep talking ab out the logician as metamathematician, and I won't keep qualifying it that way, but that's what I intend. Though I won't at all neglect the viewp oint of the working mathemati- cian, for most of this article I will b e lo oking at the leading question from the p oint of view of the logician, and for a substantial part of that, from the p ersp ective of one supremely imp ortant logician: Kurt Godel. From the time of his stunning incompleteness results in 1931 to the end of his life, Godel called for the pursuit of new axioms to settle undecided arith- metical problems. And from 1947 on, with the publication of his unusual article, \What is Cantor's continuum problem?" [11], he called in addition for the pursuit of new axioms to settle Cantor's famous conjecture ab out the cardinal numb er of the continuum. In b oth cases, he p ointed primarily to schemes of higher in nity in set theory as the direction in which to seek these new principles. Logicians have learned a great deal in recentyears that is relevanttoGodel's program, but there is considerable disagreement ab out what conclusions to draw from their results. I'm far from unbiased in this resp ect, and you'll see how I come out on these matters by the end of this essay, but I will try to giveyou a fair presentation of other p ositions along the waysoyou can decide for yourself whichyou favor. The Oxford English Dictionary de nes `axiom' as used in Logic and Mathematics, by: \A self-evident prop osition requiring no formal demon- stration to prove its truth, but received and assented to as so on as men- tioned." I think it's fair to say that something like this de nition is the rst thing wehave in mind when we sp eak of axioms for mathematics: I'll call this the ideal sense of the word. It's surprising how far the meaning of ax- iom has b een stretched from that in practice, b oth by mathematicians and logicians. Some even take it to mean an arbitrary assumption, and so refuse to take seriously what purp ose axioms are supp osed to serve. When the working mathematician sp eaks of axioms, he or she usually means those for some particular part of mathematics such as groups, rings, vector spaces, top ological spaces, Hilb ert spaces, etc. These axioms have nothing to do with self-evident prop ositions, nor are they arbitrary starting points. They are simply de nitions of kinds of structures that have b een recognized to recur in various mathematical situations. But they act as axioms in the sense that they provide a framework in which certain kinds of op erations and lines of reasoning are appropriate whereas others are not. And once weruninto a structure meeting one of these axiom systems| for example, a group asso ciated with some equation or with a top ological 2 space|we can call on a vast b o dy of previously established consequences for our further work. Without trying to argue this further, I take it that the value of these kinds of structural axioms for the organization of mathematical work is now indisputable. Moreover, weseemtokeep coming up with new axioms of this sort, and I think the case can b e made that they come up due to a continuing need to package and communicate our knowledge in digestible ways. Now, in contrast to the working mathematician's structural axioms, when the logician sp eaks of axioms, he or she means, rst of all, laws of valid reasoning that are supp osed to apply to al l parts of mathematics, and, secondly, axioms for such fundamental concepts as numb er, set, and func- tion that underlie al l mathematical concepts; I call the latter foundational axioms.Iwon't get into the question here of whether mathematics needs such axioms at all, and let the historical development of mathematics sp eak for that. Certainly, these corresp ond to such basic parts of our sub ject that they hardly need anymention at all in daily practice, and many mathe- maticians can carry on their entire work without calling on them even once. But that do esn't mean that they are not needed in the end to justify that practice, nor that they can safely b e ignored in all situations. Atany rate, I will take the necessity of foundational axioms for mathematics for granted in the following. In particular, I will b e concentrating on two axiom systems at conceptual extremes, the Dedekind-Peano Axioms for numb er theory and the Zermelo- Fraenkel axioms for set theory. I assume general familiarity with these, and so will skimp over the sp eci cs of their formulations, which are in anycase not imp ortant for the following; but I do havetosay something ab out their development and the reasons for their acceptance. In b oth cases, one started with an informal \naive" system, whichwas later transformed into a formal system in the precise sense of metamathematics. Dedekind's axioms [3] for the natural numbers N = f0; 1; 2;:: :g sim- ply to ok the initial element 0 [Dedekind started with 1] and the successor 0 op eration x 7! x (= x + 1) as basic, with the evident axioms that 0 is not a successor and that successor is one-one. Induction was formulated set- theoretically, in the form: N is the smallest set that contains 0 and is closed under the successor op eration. This takes the informal notion of arbitrary set of natural numb ers for granted, and in those terms the axioms are cate- gorical and hence complete. Dedekind used the induction principle to show that one can de ne functions by simple recursion on N, i. e., where we pre- scrib e how the function is de ned at 0 and how it is de ned at any successor 0 element x in terms of how it is de ned at x; in particular this is used in the 3 pro of of categoricity. The functions of one or more arguments from N to N generated by explicit and simple recursive de nition are nowadays called the primitive recursive functions. Peano [23] made a rst stab at adding axioms (to those of Dedekind) ab out which sets exist. He stated that every prop erty determines a set, and then gave some closure conditions on prop erties. In the Peano axioms, in- duction is equivalent to the statement that any prop ertyofnaturalnumbers that holds of 0 and is closed under successor holds of all natural numb ers. If one takes the notions of set or prop erty as not needing any further analysis, then it seems to me that the Dedekind-Peano axioms come as close as anything wehave to meeting the ideal dictionary sense of the word. But the metamathematical view is that al l notions used in an axiomatic system and al l assumptions concerning these must b e fully sp elled out. That is done by xing a formal language for numb er theory, and taking for the prop erties in the induction principle just those expressed byawell-formed formula of that language. The resulting formal system nowadays is called Peano Arithmetic and denoted PA. In the formal language of PA, whose basic relation is that of equality,=,weneedtoaddsymb ols for the op erations 0 + and to those for 0 and , with their recursive de ning equations as axioms; though + and are set-theoretically de nable in terms of the latter by Dedekind's result, they are not de nable from them in the rst-order language used for formal arithmetic.