 Physicalism is sometimes known as ‘materialism’. Indeed, on one strand to contemporary usage, the terms ‘physicalism’ and ‘materialism’ are interchangeable. But the two terms have very different histories. The word ‘materialism’ appears in English towards the end of the 17th century, but the word ‘physicalism’ was introduced into philosophy only in the 1930s by  Otto Neurath  (1931) and  Rudolf Carnap  (1959/1932), both of whom were key members of the Vienna Circle, a group of philosophers, scientists and mathematicians active in Vienna prior to World War II. While it is not clear that Neurath and Carnap understood physicalism in the same way, one thesis often attributed to them (e.g. in Hempel 1949) is the linguistic thesis that every statement is synonymous with (i.e. is equivalent in meaning with) some physical statement. But materialism as traditionally construed is not a linguistic thesis at all; rather it is a metaphysical thesis in the sense that it tells us about the nature of the world. At least for the positivists, therefore, there was a clear reason for distinguishing physicalism (a linguistic thesis) from materialism (a metaphysical thesis). Moreover, this reason was compounded by the fact that, according to official positivist doctrine, metaphysics is nonsense. Since the 1930s, however, the positivist philosophy that under-girded this distinction has for the most part been rejected—for example, physicalism is not a linguistic thesis for contemporary philosophers—and this is one reason why the words ‘materialism’ and ‘physicalism’ are now often interpreted as interchangeable. 