 Of course, these notions of definition, meaning and synonymy would themselves need to be clarified, But they were thought at the time to be sufficiently obvious notions whose clarification didn’t seem particularly urgent until W.V.O. Quine (1953 [1980a]) raised serious questions about them much later (see §3.3ff below). Putting those questions to one side, Frege made spectacularly interesting suggestions, offering a famous definition, for example, of the “ancestral” relation involved in (11) as a basis for his definition of number (see Frege’s Theorem and Foundations for Arithmetic), and inspiring the program of “logicism” (or the reduction of arithmetic to logic) that was pursued in Whitehead and Russell’s (1910–13) monumental Principia Mathematica, and the (early) Ludwig Wittgenstein’s (1922) Tractatus Logico-philosophicus.