Mind: A Brief Introduction (John R. Searle) / quote 124 of 138
Substitution and existential inference
Formally we can put this as follows: 1. [(a=b) & Fa]Fb. If a is identical with b and a has property F, then b has property F. Thus from 2. Caesar crossed the Rubicon. and 3. Caesar is identical with Mark Anthony’s best friend. we can infer 4. Mark Anthony’s best friend crossed the Rubicon. For this reason, the occurrence of “Caesar” in 2 is said to be extensional with respect to substitutability. But there are sentences in which you cannot make the substitution. Thus from 5. Brutus believes that Caesar crossed the Rubicon. and the identity statement 3, we cannot validly infer 6. Brutus believes that Mark Anthony’s best friend crossed the Rubicon, because Brutus might not believe that Caesar is Mark Anthony’s best friend. Such a sentence is said to be intensional with respect to the occurrence of Caesar. It fails the test of substitutability. The principle of existential inference says that whenever a has the property F, you can validly infer that there exists an object that has the property F. 7. Fa (x)(Fx) Thus from 8. John lives in Kansas City. we can validly infer 9. There is some x such that John lives in x. But there are sentences of this form where we cannot validly make the…