Abstract
This paper focuses on the conceptual relationship between rationality and progress as applied to scientific development. The concept of methodology is used in the sense of a model of scientific change. It is argued, as against e.g. recent discussion by Larry Laudan, that progress implies rationality. The notion of rationality useful for this conception of scientific development includes instrumental rationality, butalso the idea of rational paradigm-shift, which is analysed in terms of changes of conceptual frameworks. The key idea is the posibility of a rational reconstruction of series of conceptual frameworks, where the shift from one framework to its successor may be reconstructed as rational in the sense of there being reasons in the successor for accepting the change, and thus the whole series being rational.