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Sep 12, 2016 at 18:20 comment added Qiaochu Yuan Tim Campion's suggestion is that under mild hypotheses, categories of commutative algebras are coextensive, so their opposites are extensive. Extensivity is a categorical property that basically ensures that coproducts behave the way coproducts of spaces ought to (that they are really "disjoint" in a particular sense), so it's a reasonable candidate for a necessary condition a category should satisfy to be considered a category of "spaces."
Sep 12, 2016 at 18:18 comment added HeinrichD Do you have an answer for this? To be honest, I didn't understand the other answers (and their relation to the question).
Sep 12, 2016 at 17:39 comment added Qiaochu Yuan Sure, but why should these objects exist, and why should they relate things that look like spaces and things that look like commutative algebras?
Sep 12, 2016 at 16:50 history answered HeinrichD CC BY-SA 3.0