 a causal line may always be regarded as the persistence of something—a person, table, a photon, or what not. Throughout a given causal line, there may be constancy of quality, constancy of structure. […] That there are such more or less self-determined causal processes is in no degree logically necessary, but is, I think, one of the fundamental postulates of science. (Russell 1948: 459, quoted from Salmon 1984: 144)