 Bridgman himself was troubled by the question regarding the nature of operations and admitted late in his life that he had not really provided “an analysis of what it is that makes an operation suitable”, or “in what terms can operations be specified” (Bridgman in Frank 1956, 77). An even deeper pessimism was expressed in Bridgman’s 30-year retrospective on The Logic of Modern Physics, commissioned for Daedalus by Holton: “To me now it seems incomprehensible that I should ever have thought it within my powers … to analyze so thoroughly the functioning of our thinking apparatus that I could confidently expect to exhaust the subject and eliminate the possibility of a bright new idea against which I would be defenseless” (Bridgman 1959b, 520).