Abstract

The following Principle of Substitutivity holds for the former, but not for the latter sentence: (PS) The truth value of (the proposition expressed by) a sentence that contains an occurrence of t1 remains constant when t2 is substituted for t1, provided that t1 and t2 are codesignative singular terms. It is an undeniable fact that different sentences behave differently when it comes to which substitutions preserve their truth value. What is curious is that this fact has been presented by the philosophical tradition as a puzzle. To be more precise, what is supposed to be puzzling is the breakdown of PS in some sentences. Meanwhile, it is assumed that everything is as it should be, that nothing needs to be explained when we observe that the substitution of 'the number of planets' for 'nine' in 'nine is greater than seven' guarantees the preservation of truth value, in spite of the fact that the subject matter of the former sentence and the subject matter of 'the number of planets is greater than seven' are radically different. The former sentence expresses a claim about numbers and their relationships, whereas the latter sentence makes an assertion about our solar system.