One implication of this is that no two fermions can be in the same state. For if they were, interchanging them would not affect the total state at all, so we would have to have: –|ψ〉 = |ψ〉, i.e. |ψ〉 = 0, which is not allowed for a quantum state. This property is known as Pauli’s exclusion principle,13 and its implications for the structure of matter are fundamental. All the principal constituents of matter are indeed fermions: electrons, protons and neutrons. Without the exclusion principle, matter would collapse in on itself!
