The scientific Zeitgeist has exerted a defining influence on Boyd’s work. The following two chapters present a panorama of the scientific Zeitgeist of Boyd’s lifetime, in no small measure based on the same books Boyd read during the sixties, seventies, eighties up to the mid-nineties. Chapter 3 describes the scientific developments that occurred during the early decades of the twentieth century that Boyd incorporated in his work, and which had a tremendous influence beyond physics, from which these developments originated. In addition, the epistemological debates of the sixties will be discussed that in part arose from these developments, as well as systems theory (including the cognitive revolution), all of which have left their mark on Boyd’s work. Chapter 4 completes the study of the influence of the scientific Zeitgeist by taking a look at the development of chaos and complexity theory that took place during the late seventies, eighties and nineties, developments which Boyd closely followed and incorporated in his work (in particular in his last three presentations). It finishes with an assessment of the implications of the findings for understanding Boyd.