Subject: categories: The Yoneda axiom (was: One-stop version of my postings)

From: Vaughan Pratt <[email-masked]>
Date: Mon, 27 Mar 2006 12:05:28 -0800

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