5 Theories

Scientific theories apply new predicates to unobservable entities, like photons, to explain observable, for example optical, phenomena. How do these predicates acquire empirical meaning? Ramsey’s drastic answer in ‘Theories’ (1929) is that there are no such predicates: we use ‘is a photon’, ‘has frequency n’, and so on not as predicates but as existentially bound variables. That is, a theory tacitly starts with quantifiers, ‘properties exist – call them ‘‘being a photon’’, etc. – such that …’, followed by the explicit theory, in two parts. Its axioms link its predicate variables to each other, while its dictionary (see Campbell, N.R.) links them to observable predicates like ‘is red’ (1990a: 112). Thus if ‘a’, ‘b’ and ‘g’ are our theoretical predicates, ‘the best way to write our theory seems to be … (∃a,b,g):dictionary.axioms’ (1990a: 131). This, which is now called the ‘Ramsey sentence’ of the theory, eliminates its problematic predicates while keeping its structure and observable consequences.

Although this account has been widely accepted – and explicitly applied to functionalist theories of the mind (Lewis 1972) – its explanations of other striking facts about theories are rarely noticed. It entails for example that parts of theories, since they contain variables, are not ‘strictly propositions by themselves’ and their meaning ‘can only be given when we know to what stock of ‘‘propositions’’ … [they are] to be added’ (1990a: 131). Since this makes theoretical statements in rival theories incomparable, ‘the adherents of two such theories could quite well dispute, although neither affirmed anything the other denied’ (1990a: 133). This both explains the phenomenon of ‘incommensurability’ (see Kuhn, T.S.; Incommensurability) and limits its consequences, for example for deductive accounts of theoretical explanation (see Hempel, C.G.): for as Ramsey remarks, it does not affect reasoning within the scope of a single theory’s quantifiers.

Because Ramsey sentences say that certain universals (properties or relations) exist, nominalists, who deny this, must reject them (see Nominalism). Realists about universals, however, can use Ramsey sentences to determine what empirical universals exist, as follows. Since not only unobservable properties exist, we treat all predicates in law statements as variables. The Ramsey sentence of all such statements then quantifies over all universals that occur in laws, which are all the empirical universals there are (Mellor 1991).