The Logical Origin of Spin

Dirac combined special relativity and quantum mechanics, deriving the existence of spin—a kind of angular momentum. That angular momentum has been regarded as intrinsic, because Dirac's derivation did not try to identify whose rotation it is—let alone explain it. Following that tradition, concerning spin's physical nature, most textbooks simply declare that it is an intrinsic property, like electric charge. That declaration, while logical, is counter-intuitive, because unlike "electric charge", the term "angular momentum" already had a meaning—the momentum of rotation—long before the discovery of spin. Left unexplained, "intrinsic" is just a polite way to say "I don't know". Claiming that angular momentum is intrinsic, without saying what rotates, is equivalent to saying "that rotational property is not about rotation", which is an oxymoron. The paper is an attempt to find that physical referent.