§ 21 n. 1. See his Begriffsschrift, Halle, 1879, and Grundgesetze der Arithmetik, Jena, 1893, p. 2. ↩

§ 22 n. 1. Verbs and adjectives occurring as such are distinguished by the fact that, if they be taken as variable, the resulting function is only a proposition for *some* values of the variable, i.e. for such as are verbs or adjectives respectively. See Chap. IV. ↩

§ 25 n. 1. Schröder, Algebra der Logik, Vol. II, pp. 258-9; McColl, Calculus of Equivalent Statements, fifth paper, Proc. Lond. Math. Soc. Vol. XXVIII, p. 182. ↩

The Principles of Mathematics was written by Bertrand Russell, and published in in 1903. It is now available in the Public Domain.