In every situation I'm aware of, energy states exist because of some force that gives rise to them (dE/dx, y'know). What force did Dirac envision as giving rise to his proposed negative energy states (from which antimatter arises)? When I see it described in textbooks, the states are shown as being in increments of m0c2. If it were electromagnetic or some such, it seems to me this numerical result would not necessarily hold. I suppose I could try reading his paper. Is it available in any of the online archives?