FOM: Re: SET vs. TOP
Diskin
Diskin at aol.com
Fri Jan 23 21:15:34 EST 1998
Peter Aczel is absolutely right: FOM is indeed like a drug, and though I had
have no possibility to trace the recent hot discussion "SET vs. TOP" in the
entire details, it's very tempting to add some remarks to clarify some of the
issues (at least for myslef, I apologize in advance if they repeat something
that was already said).
Let me begin with formulating some known statement just to start. Let ZX be
the family of formal set theories of the kind "Z + something reasonable" (so
that X ranges thru F, FC, FC+CH etc). Similarly, let TX be the family of
formal theories of the kind "T + something reasonable" where T is the set of
basic topos axioms and X ranges thru NNO, Boolean condition etc.
FACT. For any theory A from ZX there is some theory B from TX in which A can
be interpreted. Conversely, any B from TX is interpretable in some A from ZX.
[]
So, I don't see any big problem in relating SET and TOP on the formal level
(the level of the pure FOM as such), if they are being thought of as ZX and TX
resp.
However, mathematics is hopefully something more than a formal game, and
behind either of the two formalisms some Natural-Set-World (NSW) is
presupposed as something independent of them (below I'll return to this
statement). NSW may appear as either some standard universe of sets, or a
family of standard universes, or a family of set-and-function universes, or
something else. What is important is that NSW does exists and so the
following question is reasonable: Which of the formalisms, SET or TOP, is a
more direct, more immediate, more suitable for human reflections about NSW?
In the terms of the FACT above, something reasonable for SET might be foreign
for TOP and conversely, so that in this informal, actually humanitarian
context, interpretability stated in the FACT again becomes an open issue.
Let me remind one very hot discussion now belonging to the history of
physics. I mean the old discussion of what is an electron: whether it's a
particle or a wave. The answer found in Copenhagen, and now widely recognized,
is that the question itself is incorrect: electron is a too complex phenomenon
to be modeled (explained) by a single mathematical formalism. In function of
the observer's goals, an electron could be successfully considered as either a
particle, or a wave (or something else, who knows?) but does not amount to
the either of models.
The same is for NSW: it is a too complex intellectual phenomenon to be
completely captured by SET or TOP separately. In some contexts SET is
preferable, in others - TOP is surely better so that only together, in some
integrated way, they give the most adequate mathematical notion of NSW at *the
present level of our knowledge*.
The reference to "the present level" is essential. Indeeed, in the above
treatment NSW appears as something Platonic. However, the Cantian view seems
to be more relevant: NSW is mainly (if not at all) our knowledge about it
rather than independent entity (though, of course, thinking it of as some
independent Platonic universe is a good working tip). After invention of the
TOP-view on sets, and its successful applications in algebraic geometry, model
theory and computer science, NSW has changed. Maybe, this explains the
boiling temperature of the discussion "SET vs. TOP": NSW of SET and NSW of
TOP are essentially different simply because in mathematics a way of thinking
something essentially contributes to constituting this thing. (Such a
relativism has recently gained some popularity even in physics where objects
of study are more independent).
In the consideration above, SET and TOP appear in a symmetric fashion. To my
mind, however, their relation is asymmetric rather than symmetric. Let me
describe one simple real life example where TOP is more preferable than SET,
it is somewhat opposed to the Harvey Friedman's example of cards on the
table.
What is the list of FOM-subscribers? It is a set varying in time, and it
seems this can be well explained in either of the two paradigms. But what
about elements of this set? In fact, they are addresses: john at aol.com,
johann at use.net etc. Note however that it may happen that john at aol.com at
some moment t1 and johann at use.net at some moment t2 are the same person.
On the other hand, john at aol.com now and john at aol.com a year later when he
studied and tried category theory in his everyday work should be considered
as different FOM-subscribers. So, while in SET the notion of element is
elementary and does not present any problem , in the real life we often deal
variable objects accessible only via also variable identification so that the
very notion of object identity becomes a highly non-trivial issue. Category
theory offers a convenient apparatus for dealing with such dynamic
collections consisting of dynamic elements, and is gradually becoming the
basic machinery of software engineering where managing such things is a must.
No doubts, all this complexity of object identification and dynamics can be
also described in the pure SET-terms. However, this would be a very bulky
specification blurring some essential aspects of the phenomenon. In contrast,
the TOP-based specification just throws light on these non-trivial questions
(I have checked this in my communication with real database designers. Note
also that, in fact, database designers and software engineers rediscovered
implicitly many of topos-theoretic concepts (in their own, terribly awkward,
terminology)).
>From the view point of the example just described, the relation between TOP
and SET is somewhat like the relation between the statistical and the
classical mechanics. Explaining dynamic objects in the SET-framework, and more
generally, explaining TOP via SET, is somewhat like that reductionism when
one says that statistic termodynamics is reducible to the classical
mechanics. On a whole, SET-motivated objections against TOP somewhat remind
me old objections against quantum mechanics and relativity from the standpoint
of the classical physics (at any rate, their temperatures are close :-).
My last remark is about standard models for TOP, the issue where it seems
there was some mess. In algebraic logic there is well known an essential
distinction between abstract algebraic models and concrete models arising from
sets. For example, for propositional calculi, this is the distinction between
abstract Boolean algebras (BA) and BA of subsets, SetBA, or abstract Heyting
algebras, HA, and Heyting algberas arising from Kripke structures, SetHA, and
so on. Similarly, for first-order predicate calculi, one should
distinguish between abstract cylindric algebras, CA, and CA arising from
sets (Kripke frames for intuitionistic logics), SetCA. Representation
theorems (like Stone's one) are always deep results showing when and how
abstract structures can be represented in concrete ones. Now, topos
structures (in fact, algebras), TA, is an algebraic version of HOL like CA is
algebraic versions of FOL. Of course, concrete TA, SetTA, do exist - they
are nothing but the presheaf toposes ( as always, concreteness is
algebraically characterized by some subdirect-irreducibility-property
correspondingly formulated, Lambek & Scott called such toposes models). Thus,
from the algebraic perspective TOP does possess standard models - these are
presheaf toposes.
Thank you for your consideration,
Zinovy Diskin
Head of Lab. for Database Design, E-mail: diskin at fis.lv
Frame Inform Systems, Ltd. Diskin at aol.com
Latvian-America Joint Venture, Phone (USA): 248 968-9511
Riga, Latvia
More information about the FOM
mailing list