FOM: Toposy-turvey
Charles Silver
csilver at sophia.smith.edu
Wed Jan 21 07:04:03 EST 1998
Solomon Feferman said:
> >[T]he notion of topos is a relatively sophisticated mathematical
> >notion which assumes understanding of the notion of category and that in
> >turn assumes understanding of notions of collection and function.
Colin Mclarty answered:
> To me, understanding the notion of category is the best way
> known today of understanding the notions of "collection and function".
>
> To be precise: Pace Feferman's theory, the first thing you
> need to know about collections and functions is that a pair of functions
> may have a composite, and this composition is associative when it is
> defined. And now I have completely stated the category axioms. This
> seems to me not very sophisticated mathematics. The topos axioms are
> more sophisticated, but no more so than ZF or Feferman's theory of
> collections and functions.
I want to raise some elementary concerns about the picture of
category theory under consideration. First, I'll present a very basic,
perhaps very naive picture of set theory:
Almost all beginning set theory textbooks get off the ground by
stating that the central concept is that of a "collection," an
"assemblage," an "ensemble," etc. On these accounts, the beginning notion
is one that we are fairly familiar with in real life, via counting and so
on. "Set," as I understand it, is a technical term pertaining to set
theory. The idea is (or one of them, anyway) that a somewhat familiar,
but imprecise concept ("collection") is being captured or merely defined
by the precise axioms of set theory. Thus, the ordinary concept of a
collection is turned into the precise one of a set. To say exactly what
sets are, various axioms are provided. And, there doesn't seem to be much
disagreement about the cumulative hierarchy. Further on, what properties
constitute a set are in some dispute. And, there is dispute about whether
the notion of "set" is such as to determine various properties, such as
CH, etc.
I won't go on with the above, naive picture. I just wanted to
provide a background for some comments and questions. According to the
above, "set" is a foundational concept that derives from the ordinary
concept of "collection" that we're familiar with in counting, and so on.
Fundamental to the notion of a collection is the concept of something
being an element or not of that collection. As I understand it, this is
all that is needed to start to mold (informal) collections into (formal)
sets. To me, this is an appealing picture. There are aspects of the
picture I presented above that I feel a bit uncomfortable with, but I'd
like to skip my doubts about the set-theoretic account I sketched and
ask whether someone partial to category theory can explain the foundations
of category theory at the same elementary level that I explained the
foundations of set theory.
I have followed Colin Mclarty's and others' very interesting and
informative explanations about toposes, but I still don't grasp some of
the basic ideas. For example, if category theory takes as basic the
notion of "function," could someone please explain in a very basic way how
the category-theoretic notion of "function" derives from the ordinary
notion of "function," on a par with the way "set" derives from
"collection"? Let me call the technical notion of function in category
theory `Function_C', and I'll stick with `function' for our ordinary
notion (vague as that notion is). I can see that it is pretty easy to
give a *formal* definition of Function_C. The formal stuff doesn't bother
me. What I don't see is how Function_C relates to function. That is, how
does the formal definition arise from the ordinary notion? In set theory,
I can see that much (but not all) of the formal account of sets arises
from the ordinary account of collection. From the postings on category
theory so far, I don't have this understanding for category theory. I
realize that in asking these elementary questions I may be straining the
patience of Colin and others who have so patiently responded to a
multitude of challenges. But, I honestly don't get it. If someone would
attempt to answer my elementary questions, I would greatly appreciate it.
Thank you,
Charlie Silver
Smith College
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