V. SUDHAKAR* Are Scientific Theories Incommensurable ? The normal-scientific tradition that emerges from a scientific revolution is not only incompatible but 'often actually incommensurable with that which has gone before —Thomas S. Kuhn1 Knowledge is...as ever increasing ocean of mutually incompatible (and perhaps even incommensurable) alternatives. —Paul K. Feyerabend2 Can we accept the above said incommensurability thesis of Thomas Kuhn and Paul Feyerabend concerning the nature of the relation between successive scientific theories ? This paper is atleast a partial answer to this question, and the discussion in this is primarily centred around three main issues : (i) the Positivist and Popperian model of scientific change, (ii) Kuhn & Feyerabend ideas on scientific change and (iii) the contemporary developments and alternatives against these two conceptions. Before entering into the full-fledged debate on this question a few words about theory change are in order. Sensitivity to the fact that science is primarily a diachronic activity in generating very 'general' and 'specific' ideas and conceptions provides an intelligible picture of scientific change and the historicity of science. In the course of history scientists change their assumptions, definitions, statements, beliefs, explanations, methods, values, aims and goals, criticisms and evaluation criteria, etc. All these changes in scientists thought processes and practical activities are nothing more than a set of changing body of beliefs about the Nature. Let us suppose a scientist (or a group of scientists) changes his belief of a theory Ti to T2, then the important question here is why did he change his belief from Ti to T2 ? Is it because T2 is better than Ti, interms of its truthnearness or adequacy ? Or is belief in Tz more rational than Ti interms of its consistency or coherence or evidence available at that particular moment of time ? In other words does the scientist change his belief on rational grounds ? If a scientist refuses to change his belief from Ti to T2 inspite of the new evidence available then is he irrational or non-rational ? In general, on what ground theory change takes place, if at all there are any rational criteria, universal or relative ? Or are there any rational theory choices among the community of scientists ? These are some of the seminal questions raised in the recent past in the literature of history and philosophy of science, philosophy of language, sociology of science and also in the area of * Lecturer, Department of Education, Osmania University, Hyderabad.