The essence of Mach’s philosophy was this, in Einstein’s words: “Concepts have meaning only if we can point to objects to which they refer and to the rules by which they are assigned to these objects.”85 In other words, for a concept to make sense you need an operational definition of it, one that describes how you would observe the concept in operation. This would bear fruit for Einstein when, a few years later, he and Besso would talk about what observation would give meaning to the apparently simple concept that two events happened “simultaneously.”
