the.com/set theory

means The branch of mathematics that studies collections of objects (called sets) and the rules for how they relate, contain, and combine — it's the foundational language that much of modern math is built on top of.

from The term comes from 'set' in its ordinary English sense of 'a group of things belonging together,' paired with 'theory' from Greek 'theoria' (contemplation, a way of looking at something). The mathematical idea took shape in the late 1800s, largely through Georg Cantor's work on comparing infinities — he needed a rigorous way to talk about collections, including infinite ones, and 'set' (German 'Menge') became the basic unit. The 'bags of nothing' flavor comes from the fact that formal set theory can build every set, including numbers themselves, starting from literally nothing — the empty set — nested inside itself over and over.