Pauli exclusion principle

The quantum mechanical principle, formulated by Pauli, that no two identical fermions may occupy the same quantum state.

Fermions of the same species form totally antisymmetric states. In the two-particle case:

|ψψ′> = - |ψ′ψ>

If both particles occupy the quantum state |ψ>, the state of the entire system is |ψψ>. Then

|ψψ> = - |ψψ> = |0>

so such a state does not occur. This is readily generalizable to the case of more than two particles.

The Pauli exclusion principle is the reason you do not fall through the floor.