quantum mechanics

In quantum mechanics, the Pauli exclusion principle states that two or more identical particles with half-integer spins (i.e. fermions) cannot occupy the same quantum state within a quantum system simultaneously.
Pauli exclusion principle
Quantum mechanics (Wikipedia) Quantum mechanics is a fundamental theory in physics that describes the behavior of nature at the scale of atoms and subatomic particles. It is the foundation of all quantum physics including quantum chemistry, quantum field theory, quantum technology, and quantum information science.
Schrödinger equation
Theoretical physicists live in a classical world, looking out into a quantum-mechanical world. John Stewart Bell
Von Neumann was the first to establish a rigorous mathematical framework for quantum mechanics, known as the Dirac–von Neumann axioms, in his influential 1932 work Mathematical Foundations of Quantum Mechanics.
wave–particle duality