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## Main Question or Discussion Point

The principle states that no two identical fermions in a system can be in the same quantum state, but what I don't fully understand is how you define a "system". For example when you apply statistical thermodynamics to a gas of non-interacting fermions you say that a maximum of one can occupy each single particle state. Maybe I'm confused or forgetting something important but I don't see why you couldn't consider all fermions of a particular kind in the universe as a system of non-interacting particles in the same way and conclude that none of them can share a single particle state.