MathOverflow is a question and answer site for professional mathematicians. It's 100% free, no registration required.

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

MacLane, as well as probably any other category book, does not hesitate to define a product of two categories as a category consisting of pairs of objects, etc.

Now my question is: what law of nature or logic or anything allows to create such pairs? Pair creation may be an axiom, say, in Set Theory. In category theory there's no such thing; they seem to just fall from heaven, keeping in mind that category theory is not based on sets at all. It looks pretty suspicious to me; but maybe I'm wrong.

Any comments?

P.S. An even curiouser question is about disjoint union of two (non-small) categories.

share|cite|improve this question

closed as off-topic by Eric Wofsey, Noah Snyder, Stefan Kohl, Benjamin Steinberg, Ricardo Andrade Jun 15 '14 at 1:44

This question appears to be off-topic. The users who voted to close gave this specific reason:

  • "MathOverflow is for mathematicians to ask each other questions about their research. See Math.StackExchange to ask general questions in mathematics." – Eric Wofsey, Stefan Kohl, Benjamin Steinberg, Ricardo Andrade
If this question can be reworded to fit the rules in the help center, please edit the question.

Category theory is very much based on sets! Who told you otherwise? – Eric Wofsey Jun 14 '14 at 6:20
There is a notion of product in a bicategory, it is defined similarly to products in categories, but with 2-morphisms implementing higher coherence data. The bicategory of all categories admits all products and coproducts (and in fact all limits and colimits). Before Lawvere characterized the category of sets as a well-pointed elementary topos with a natural numbers objects he tried to characterize the category of all categories in a similar way, though as far as I understand his classification did not succeed because he used categories instead of bicategories. – Dmitri Pavlov Jun 14 '14 at 6:22
@EricWofsey not all formulations of category theory require sets. – Tim Seguine Jun 14 '14 at 11:10
Eric Wofsey, category theory is not based on sets. Or else there would be a different category theory for each set theory. Btw, I hope you do not imply set theory is based on set theory. – Vlad Patryshev Jun 14 '14 at 13:23
@VladPatryshev: I have no idea what your comment means. I also don't really know what your question means. What do you mean when you say "in category theory"? When most people talk about category theory, category theory isn't something that just stands alone; it's formulated within some foundation for mathematics just like any other part of math. Pretty much any foundation for mathematics involves an axiomatization of something that can reasonably be called "sets". – Eric Wofsey Jun 14 '14 at 20:18

My copy of Mac Lane's book is a few thousand miles away from me at the moment, so the following is based on possibly faulty memory, but here goes anyway: Mac Lane works in a set-theoretic foundational system, something like ZFC plus one Grothendieck universe. So he has no problem with the existence of ordered pairs.

There are people who want to use categories as a foundational system, without any reliance on set theory. These people need to design their foundational system so as to ensure the existence of products of categories and lots of other things that would ordinarily be provided by a set-based foundation.

share|cite|improve this answer

Not the answer you're looking for? Browse other questions tagged or ask your own question.