 Feyerabend therefore idolised semantic instability, arguing that the semantic stability presupposed by positivist accounts of reduction, explanation and confirmation, has been and should be violated if we want progress in science. If meaning is determined by theory, terms in very different theories simply cannot share the same meaning: they will be “incommensurable”. Any attempt to derive the principles of an old theory from those of a new one must either be unsuccessful or must effect a change in the meaning of the old theory’s terms. The “theoretical reduction” beloved of Logical Empiricists is therefore actually more like replacement of one theory and its ontology by another. At the end of his well-known 1962 paper “Explanation, Reduction, and Empiricism”, in which he introduced the concept of incommensurability, Feyerabend concluded that this concept precluded any formal account of explanation, reduction or confirmation. (Kuhn’s book The Structure of Scientific Revolutions, in which the same term was used to characterize a related concept, was published in the same year).