"It thus appears that we must abandon the identification of the holes with protons and must find some other interpretation for them. A hole, if there were one [in the world], would be a new kind of particle, unknown to experimental physics, having the same mass and opposite charge to an electron. We may call such a particle an anti-electron. We should not expect to find any of them in Nature, on account of the rapid rate of recombination with electrons, but if they could be produced experimentally in high vacuum they would be quite stable and amenable to observation. An encounter between two hard γ-rays (of energy of at least half a million volts) could lead to the creation simultaneously of an electron and anti-electron. This probability [of the creation of a pair] is negligible, however, with the intensities of γ-rays at present available."