1. Black, Max. 1964. A Companion to Wittgenstein’s Tractatus. Ithaca, NY: Cornell University Press.
2. Frege, G. 1879. Begriffsschrift, eine der arithmetischen nachgebildete Formelsprache des reinen Denkens. Halle a. S.: Louis Nebert
3. translated as Concept Script, a Formal Language of Pure Thought Modelled Upon that of Arithmetic, ed. S. Bauer-Mengelberg, and J. van Heijenoort. From Frege to Gödel: A Source Book in Mathematical Logic, 1879-1931. Cambridge, MA: Harvard University Press, 1967.
4. Frege, G. 1918. Der Gedanke. Eine Logische Untersuchung. Beiträge zur Philosophie des deutschen Idealismus, I (1918-1919): 58-77
5. translated as 'Thoughts', P. Geach, and R. Stoothoff, in ed. McGuinness. 1984, pp. 351-372.