👉 Levins, also known as Levenson's or Levins' lemma, is a fundamental theorem in mathematical logic and set theory that deals with the existence of infinite sets. It states that if there exists an infinite set A such that for every infinite subset B of A, either B has exactly one element or B is equal to A itself, then A must be countable. In other words, if we consider a set A as infinitely large (i.e., its cardinality grows without