 Alternatively, following D. Lewis 1973, we might understand determinism as the thesis that our world is governed by a set of natural laws which is such that any two possible worlds with our laws which are exactly alike at any time are also exactly alike at every other time (see also Earman 1986). This second definition of determinism is stronger than the first; if a possible world is deterministic according to the Lewis/Earman definition, it is deterministic according to the Entailment definition, but not vice versa.