Vorlesungen über Physik, Band I, Kap. 12.1 (Übersetzung: Heinz Köhler), Seite 165, Oldenbourg München Wien, 5. Aufl. 2007
Original englisch: "The trick is the idealizations. [...] This system is quite unlike the case of mathematics, in which everything can be defined, and then we do not know what we are talking about. In fact, the glory of mathematics is that we do not have to say what we are talking about. The glory is that the laws, the arguments, and the logic are independent of what 'it' is." - The Feynman Lectures on Physics: Quantum Mechanics. Addison-Wesley Pub. Co., 1965.