 In Weyl (1949), Carnap (1958), Feyerabend (1962) and a number of other writings, we find different formulations of the idea that the axioms of a scientific theory determine the meaning of theoretical terms without these axioms qualifying as proper definitions of theoretical terms. Lewis, however, wrote a paper with the title “How to Define Theoretical Terms” (1970). A closer look at the literature reveals that the very idea of explicitly defining theoretical terms goes back to Carnap’s (1961) use of Hilbert’s epsilon operator in scientific theories. This operator is an indefinite description operator that was introduced by Hilbert to designate some object x that satisfies an open formula \(\phi\). So