 While the principle theories-constructive theories distinction first made its way into print in 1919, there is considerable evidence that it played an explicit role in Einstein’s thinking much earlier (Einstein 1907, Einstein to Sommerfeld 14 January 1908, CPAE, vol. 5, Doc. 73, Einstein 1914). Nor was it only the relativity and light principles that served Einstein as constraints in his theorizing. Thus, he explicitly mentions also the Boltzmann principle, \(S = k \log W\), as another such: