 Feyerabend drew heavily from Duhem’s The Aim and Structure of Physical Theory (1954 [1906]) in his development of the notion of incommensurability of scientific theories. Many of the main points Feyerabend emphasized by calling scientific theories incommensurable had been developed already by Duhem, who had argued that logic is insufficient for determining the outcome of theoretical disputes in the natural sciences, and who documented the difficulties historians have in understanding the development of the natural sciences due to meaning change. Duhem also already highlighted the communication difficulties between proponents of competing scientific theories because of these differences in meaning. For example, Duhem had claimed that what a physicist states as the result of an experiment is not simply the recital of some observed facts. Rather, it is the interpretation of these facts on the basis of the theories the scientist regards as true (1954 [1906], 159). It follows, according to Duhem, that in order to understand the meanings that scientists ascribe to their own statements, it is necessary to understand the theories that they use in order to interpret what they observe. Thus, Duhem had stated an early version of Feyerabend’s incommensurability thesis. Moreover, Duhem explicitly limited his discussion to non-instantial, physical theories, as opposed to mere experimental laws. This is very similar to the criteria that marks the most significant difference between Kuhn and Feyerabend’s development of the idea of incommensurability (see Section 4), and that also delimits Einstein’s use of ‘incommensurable’ while discussing the problems of theory comparison (see Section 3.2.3).