Tibor Machan makes some obvious but important points about multiple universes. One is that there cannot be two or more universes if by 'universe' is meant everything that exists in spacetime. I would add that this is a very simple conceptual truth, one that we know to be true a priori. It lays down a contraint that no empirical inquiry can violate on pain of tapering off into nonsense. So talk of multiple universes, if not logicaly contradictory, must involve an altered, and restricted, use of 'universe.' But then the burden is on those who talk this way to explain exactly what they mean.
Philosophers often speak of possible worlds. There is nothing problematic about there being a plurality of possible worlds, indeed an infinity of them. But there is, and can be, only one actual world. The actual world is not the same as the physical universe. For not everything actual is physical. My consciousness is actual but not physical. A second reason is that the actual world is a maximal state of affairs, the total way things are. It is a totality of facts, not of things, as Ludwig the Tractarian once wrote. But the physical universe is a totality of physical things not of facts.
For more see Some Theses on Possible Worlds.
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