 To help explicate incommensurability in terms of taxonomic classification, Kuhn developed the no-overlap principle. The no-overlap principle precludes cross-classification of objects into different kinds within a theory’s taxonomy. According to the no-overlap principle, no two kind terms may overlap in their referents unless they are related as species to genus. For example, there are no dogs that are also cats; no gold that is also silver, and that is what makes the terms ‘dogs’, ‘cats’, ‘silver’, and ‘gold’ kind terms (Kuhn 2000 [1991], 92). Such kind terms are used to state laws and theories and must be learned together through experience (2000 [1993], 230; cf. Barker et al. 2003, 214 ff.). There are two possibilities. Most kind terms must be learned as members of one or another contrast set. For example to learn the term ‘liquid’, one must also master the terms ‘solid’ and ‘gas’. Other types of kind terms are not learned through contrast sets, but together with closely related terms through their joint application to situations that exemplify natural laws. For example, the term ‘force’ must be learned together with terms like ‘mass’ and ‘weight’ through an application of Hooke’s law and either Newton’s three laws of motion or else the first and third laws together with the law of gravity (2000 [1993], 231). According to Kuhn, scientific revolutions change the structural relations between pre-existing kind terms, breaking the no-overlap principle (2000 [1991], 92–96]. This is to say that theories separated by a revolution cross-classify the same things into mutually exclusive sets of kinds. A kind from one taxonomy is mutually exclusive with another if it cannot simply be introduced into it because the objects to which it refers would be subject to different sets of natural laws. This would result in conflicting expectations about the same objects, loss of logical relations between statements made with those concepts, and ultimately incoherence and miscommunication (Kuhn 2000 [1993], 232, 238). For example, Ptolemy’s theory classifies the sun as a planet, where planets orbit the earth, while Copernicus’ theory classifies the sun as a star, where planets orbit stars like the sun. A correct statement according to Copernican theory, such as “Planets orbit the sun” is incoherent in Ptolemaic vocabulary (2000 [1991], 94). Moreover, the Copernican claim that the planets orbit the sun could not even have been made without abandoning the Ptolemaic concepts and developing new ones to replace (and not supplement) them.