- Bijection(in set theory and mathematics)
- A perfect one-to-one matching between two sets, where every element in one set pairs with exactly one element in the other, and nothing is left unpaired.
- Jensen(in mathematical logic)
- Ronald Jensen is a mathematical logician who developed important ideas in set theory, building on work by other mathematicians and philosophers.
- NF(Formal set theory)
- A set theory (New Foundations) with a stratified comprehension scheme, from which no contradictions are currently known to follow but which has uncomfortable consequences including the failure of the Axiom of Choice
- NFU(in set theory)
- A modified version of Quine's NF system that Jensen created; it adds extra objects called 'urelements' to make the system work more flexibly.
- Quine(as a proper name referring to the philosopher whose theory is being discussed)
- Willard Van Orman Quine was a 20th-century American philosopher who wrote about how we know things and how language works. In this statement, we're discussing one of his specific ideas about observation.
- Stratification(in formal logic and set theory)
- In Quine's logic, a property of formulas where you can assign levels or ranks to variables in a way that respects the rules of the system—essentially, a formal way of keeping things organized and consistent.
- Urelements(in set theory)
- Basic objects in set theory that aren't sets themselves—they're the fundamental building blocks, like atoms, that other sets are built from.
- cardinality(Central to comparing infinite sets and establishing that no universal set exists.)
- A measure of the size of a set, indicating the number of elements it contains.
- ℘₁(V)(in mathematical notation)
- A mathematical notation meaning 'the power set of V,' which is the collection of all possible subsets (smaller groups) that can be made from the set V.