Robert J. MacG. Dawson wrote:

Vaughan Pratt wrote:

...

In axiomatic mathematics, everything that is not forbidden is permitted.

Yes, in a sense... but theorems along the lines of "there exists a model of X such that Y" were a long, long way in the future <grin>

Reuben Hersh, cited in Eugenia Cheng's fascinating paper "Mathematics, Morally," put it succinctly in 1991. "There is an amazingly high consensus in mathematics as to what is 'correct' or 'accepted'." It's really too bad we can't go back and show those old-timers what their stuff has evolved into. Most of it would presumably evoke the ancient counterpart of "Wow," but that's boringly predictable. What would really be interesting would be the less predictable reactions of the form "I knew that but didn't write it down because...". For example the basic concept of a model must surely have occurred to many over the millennia but seemed pointless formulating as a mathematical concept in its own right, perhaps for want of the concept of any alternative model to that one, perhaps for want of a suitable framework, perhaps because everyone else seemed to be taking it for granted as part of the ambient logic so why should I rock that boat? One can see this in the late Robert Floyd's seminal 1967 paper "Assigning meanings to programs" on the verification of imperative programs, where he clearly has some model of his axioms in mind in his completeness proof for his axiomatization of assignment x:=e (the dual of Hoare's subsequent axiomatization), yet it does not occur to him to say what the model is. In my 1976 paper "Semantical considerations on Floyd-Hoare logic" I made explicit the model Floyd apparently had in mind in his proof by defining it to be a Kripke structure (of the kind introduced by Kripke in "Semantical considerations on modal logic"), showed that Floyd's informal completeness argument worked just fine when formalized for that model (so Floyd was morally correct), and extended it to a complete Floyd-style axiomatization of array assignment (whose completeness is complicated by the possibility of aliasing as in a[a[i]]:=a[a[j]]). (The paper then went on to define modal dynamic logic, which I put at the end as merely a syntactic sidebar of marginal interest; unexpectedly it caught people's fancy and appears to have settled down as a staple of linguistic theory.) I believe this was the first application of model theoretic methods to the verification of programs containing assignment statements, the kind of program forming the mainstream of programming languages. (Functional programming in contrast was designed from the outset to support semantics, whence its popularity with logicians.) It certainly was the first to use Kripke structures for that purpose, which today are the standard model for imperative programming. The paper cites and treats a number of references to prior applications of modal logic to programming, none of which however offered a semantics, seemingly having such a semantics only in the back of their mind as with Floyd. This neglect of semantics continues even today: most programmers, along with their managers, remain oblivious to the important backroom role of semantics in the verification of their programs, to the extent of being unaware even of its existence. A basic reason for not writing down what one is thinking is lack of awareness of one's thought processes. Another is want of a suitable framework for formulating those thoughts in a way other than as what later mathematicians would judge as handwaving. Yet another is the difficulty, even today, of separating mathematics from religion, defined as matters of faith. The irrationality of the diagonal of the unit square was at one point unthinkable, giving us no way of knowing for how many centuries it was a dark secret of certain mathematical cults bordering on heresy or worse. Today's mathematical religions have shifted their attention away from existence, where nowadays everything goes (though Robinson-style infinitesimals are a tiny pill that some find hard to swallow), instead focusing on relevance, utility, and other subjective criteria for acceptance. Foundations reared its head in the 19th C, forcing mathematicians to hold up their intuitions to the bright lights and microscopes of logic. This process is somewhat akin to fashion designers being asked to appraise the apparel of their emperor. Once a consensus has built, even if artificially as necessary for underdressed emperors, it becomes very hard to stick to your guns. This phenomenon makes it impossible to tell, post-Cantor, whether those intuitions were more set theoretical or category theoretical in nature. Answers to that must be sought in pre-Cantorian mathematics. My personal belief is that when the pathways of the human brain are finally unravelled and understood, they will be shown to be based on Yoneda, not as a lemma however but as an axiom postulating a certain dense embedding. In the millions of years of evolution of primate thinking, no productive mathematical mechanism has a higher probability of being stumbled on than mathematics founded on the Yoneda axiom. I know of no better explanation of how human thought could have evolved to its present form than evolution finding and exploiting the Yoneda principle, that (for example) the unique 2-element semigroup whose Cayley table has distinct constant columns can generate every graph and graph homomorphism in a simple, uniform, and economical way. The likelihood of evolution stumbling on a serviceable fragment of Zermelo's axioms other than by our conscious thought seems considerably lower. If evolution discovered both, chances are it found Yoneda first. Yoneda needs so much less neural machinery than any of its competitors. Yet even today Google has not stumbled on the Yoneda axiom (as a phrase). I put that down to the mechanism being such a fundamental part of how we think that we have no more mental access to it than we have physical access to quarks. Proof that our brains incorporate the Yoneda axiom will have to await more advanced technology, until then that model of the evolution of human thought will remain a conceit of the cracks and pots promoting it. Vaughan Pratt