 Changes in problems and standards come with corresponding conceptual changes, so that after a revolution, many (though not all) of the older concepts are still used, but in a slightly modified way. Such conceptual changes have both intensional and extensional aspects, which is to say that the same terms take on different meanings and are used to refer to different things when used by proponents of competing incommensurable theories. The changes in the intensional aspects of concepts result because the terms used to express a theory are inter-defined and their meanings depend on the theories to which they belong. For example, the meanings of the terms ‘temperature’, ‘mass’, ‘chemical element’ and ‘chemical compound’ depend on which theories are used to interpret them. Conceptual changes also result in the exclusion of some old elements of the extension of a concept, while new elements come to be subsumed by it, so that the same term comes to refer to different things. For example, the term ‘Planet’ referred to the sun but not the earth in the Ptolemaic theory, whereas it refers to the earth and not the sun in the Copernican theory. Incommensurable theories use some of the same terms, but with different meanings, to refer to different sets of things. Two scientists who perceive the same situation differently, but nevertheless use the same vocabulary to describe it, speak from incommensurable viewpoints (Kuhn 1970, 201).