David Lewis argued every possible world is exactly as real as this one

Philosopher David Lewis defended 'modal realism': when we say something is merely possible, we aren't speaking loosely — a concrete world where it actually happens really exists, just as solid as ours, only causally cut off from it. On this view unicorns aren't fictions but facts, true somewhere among infinitely many worlds. Almost every philosopher calls the idea wildly counterintuitive, and then struggles to refute it.

— David Lewis, On the Plurality of Worlds — 1986
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