1. R. P. Feynman, R. B. Leighton and M. Sands, The Feynman Lectures in Physics 1 (Addison-Wesley, Reading, 1963) pp. 1–2.
2. R. P. Feynman, QED: The Strange Theory of Light and Matter (Princeton University Press, Princeton, 1985) p. 12.
3. Foundations of Quantum Mechanics I