Two simple considerations have been given inadequate or no attention concerning the incommensurability thesis introduced by Kuhn (1962) and by Feyerabend (1962) and later given a somewhat different form by Lakatos (1968Lakatos ( , 1970a, b), b).
The Kuhn-Feyerabend thesis is this. Two theories are incommensurable (e.g., the Newtonian and Einsteinian theories of motion), if they contain a basic common term whose meaning or use in one theory is incommensurable with its meaning or use in the other, i.e., if at least one basic term used in both theories has a totally different meaning in each. Incommensurability renders it impossible to compare or contrast, relate, or otherwise discuss the content of the two theories. They differ in relation to the problems they solve or give rise to; but no scientific/rational procedure plays a decisive part, or perhaps even an influential part, in deciding between them. Conventionalism of one sort or another is a natural sequel. (In commensurability could equally be a consequence of conventionalism; according the Skolimowski (1967), the notion of incommensurability resulting from conventionalism stems from a paper published in 1934 by Ajdukiewicz.)
Simple considerations about the incommensurability thesis might have shown in advance that it must be untenable. Hattiangadi has pointed out that, even if the acceptance of a theory depends on the scientist's assessment of what problems are solved by the alternative theories available, observational tests do exist that discriminate between the exemplars of supposedly incommensurable theories, Newton's and Einstein's; thus the three classical tests discriminate fairly sharply between them, and hence decide scientifically/rationally between them. 1 Even sharper, however, is the following. The metric or, more simply the notion of a length is held to have a different meaning in Einsteinian mechanics from Galilean mechanics: in the one, it is a three-termed relation between the distance to be measured, the measuring-rod, and the