Wolfgang Pauli, at ETH Zurich, together with his young assistant Victor Weisskopf, publishes in 1934 "Über die Quantisierung der skalaren relativistischen Wellengleichung" ("On the Quantization of the Relativistic Scalar Wave Equation") in Helvetica Physica Acta. Pauli felt strong distaste for Dirac's hole theory — the framework that explained the positron as a "hole" in an infinite sea of occupied negative-energy states — and together with Weisskopf seeks to show that equivalent physics can be reached without resorting to that idea, applying instead the method of second quantization to the Klein-Gordon equation, which describes spin-zero particles (bosons), not spin-1/2 particles like Dirac's electron. Pauli would later call this theory, with some irony, the "anti-Dirac theory". The result, however, ends up being deeply complementary to Dirac's rather than replacing it: Pauli and Weisskopf show that, when correctly quantizing the charged scalar field, particles with opposite charges to their corresponding antiparticles naturally appear, exactly as happened with the electron and the positron — but now for bosons, with no recourse whatsoever to holes in an infinite sea of occupied states. This is a conceptually decisive result: it proves that the existence of antiparticles is not an exclusive peculiarity of spin-1/2 fermions, but a general and necessary feature of any relativistic quantum field theory. The work also corrects the then-widespread idea that incorporating the principles of special relativity into quantum mechanics necessarily required spin-1/2 particles. The Klein-Gordon equation, until then problematic for admitting negative probability densities in its single-particle interpretation, is thus successfully reinterpreted in the language of quantum fields, laying the groundwork for the modern treatment of scalar particles — including, decades later, the Higgs boson.