 In the ground-breaking central papers from this period of Feyerabend’s oeuvre such as “How to be a Good Empiricist” (1963), “Realism and Instrumentalism” (1964), “Problems of Empiricism” and “Reply to Criticism” (1965), his most important argument for scientific realism was methodological: realism is desirable because it demands the proliferation of new and incompatible theories. This leads to scientific progress because it results in each theory having more empirical content that it otherwise would, since a theory’s testability is proportional to the number of potential falsifiers it has, and the production of alternative theories is the only reliable way to ensure the existence of potential falsifiers. So scientific progress comes through “theoretical pluralism”, allowing a plurality of incompatible theories, each of which will contribute by competition to maintaining and enhancing the testability, and thus the empirical content, of the others. According to Feyerabend’s pluralistic test model, theories are tested against one another. He thus idealised what Kuhn called “pre-paradigm” periods and “scientific revolutions”, occasions when there are many incompatible theories, all forced to develop through their competition with each other. But he downplayed the idea that theories are still compared with one another primarily for their ability to account for the results of observation and experiment. For Feyerabend, this idea was an empiricist myth which disguised the role of aesthetic and social factors in theory-choice.