The historical study of 20th century field theories in the preceding chapters provides an adequate testing ground for models of how science develops. On this basis I shall argue in this chapter that one of the possible ways of achieving conceptual revolutions is what I shall call “ontological synthesis” (Section 12.4). This notion is based on, and gives a strong support to, a special version of scientific realism (Sections 12.3 and 12.5). It has also provided a firm ground for the rationality of scientific growth (Section 12.6).