math.LO math.LO

In the world of logic, there is a bridge. It spans the chasm between a rule's admissibility and its derivability from axioms. As long as rules involve only a finite number of premises, the bridge holds firm — for classical logic, anything that doesn't violate truth is already contained in the syntax. But add infinite premises, and the supports start to tremble. The bridge, alas, is not all-powerful.

In mathematics there is a sieve that sifts not dust, but infinity itself. It’s called a Q-space: here any subset turns out to be a Gδ set — a countable intersection of open sets, as if sifted through an unimaginably fine mesh.