The happiest thought of Einstein's life was when he imagined falling freely in space, thus fathoming the equivalence principle (Pais 1982, 178). Albert Einstein (18791955) was a master at performing thought experiments and performed experiments in the material sense as well, as illustrated by the Einstein/de Haas experiments (Galison 1987, 34-52; also Hughes 1993). Moreover, he calculated and speculated as a theoretical physicist. Thus he satisfies the criteria which the philosopher of science Ian Hacking has proposed to characterize theoretical practice. An ideal theorizer calculates, experiments and speculates. The examples Hacking adduces in Representing and Intervening are Newton and Laplace, which nearly fits Hacking's bill (Hacking 1983, 212-215). The mathematican Hermann Weyl (1885-1955) also fits these criteria generously interpreted, for he speculated, calculated and performed thought experiments. In addition, what characterizes many theorizers are their expository gifts which all these scientists have in common, not the least Weyl.