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Principle/Nozick: to show that principles explain a p, involves that they contain it. But that does not prove that p.
Richness/principle/existence/Nozick: thesis: "All possibilities are realized." - This follows from the assumption of the egalitarian theory that the options "something"/"nothing" are equal. - This requires infinitely separate possible worlds because options can be contradictory. - Then you need no explanation why something is or is not, because everything is (somewhere) realized. - Then there is no fact "X instead of Y".
Nothing: one of the unrealized possibilities is also that there is nothing - but that is one among many, not the inegalitary situation that there would be "exclusively nothing".
Consciousness/explanation/evolution theory/Nozick: consciousness allows other types of behavior: - to be guided by principles.
Philosophical Explanations Oxford 1981
The Nature of Rationality 1994