Ludwig Wittgenstein (1974). “Philosophical Grammar: Part I, The Proposition, and Its Sense, Part II, On Logic and Mathematics”, p.66, Univ of California Press
Ludwig Wittgenstein, G. E. M. Anscombe, Linda L. McAlister, Margarete Schättle (2007). “Remarks on Colour/Bemerkungen Uber Die Farben”, Univ of California Press