 For Einstein, as for many others, simplicity is the criterion that mainly steers theory choice in domains where experiment and observation no longer provide an unambiguous guide. This, too, is a theme sounded early and late in Einstein’s philosophical reflections (for more detail, see Howard 1998, Norton 2000, van Dongen 2002, 2010, Giovanelli 2018). For example, the just-quoted remark from 1918 about the apparent determination of theory choice in practice, contrasted with in-principle underdetermination continues: