Propositional attitude statements – statements about our beliefs, desires, hopes and fears – exhibit certain logical peculiarities. For example, in apparent violation of Leibniz’s law of the indiscernibility of identicals, we cannot freely substitute expressions which designate the same object within such statements. According to Leibniz’s law, every instance of the following scheme is valid:
- a = b
- F(a)
- Therefore, F(b)
The validity of Leibniz’s law seems beyond question. It says, in effect, that if an object has a certain property, then anything identical to that object also has that property. Valid instances abound. But consider the following apparently invalid instance:
- Hesperus is Phosphorus
- Hammurabi believed that Hesperus often rose in the evening
- Therefore, Hammurabi believed that ‘Phosphorus’ often rose in the evening.
If we take ‘Hammurabi believed that…often rose in the evening’ to serve as the predicate F and ‘Hesperus’ and ‘Phosphorus’ to be a and b respectively, this argument appears to be an instance of Leibniz’s law. Yet (3) apparently fails to follow from (1) and (2). Hammurabi believed that Hesperus and Phosphorus were two heavenly bodies not one. And he believed that Hesperus did, but that Phosphorus did not rise in the evening.
We have derived a false conclusion from true premises and an apparently valid law. If that law is really valid, then our argument had better not be a genuine instance of the law. The tempting conclusion, widely accepted, is that we were wrong to construe propositional attitude statements as simple predications. We should not, that is, construe ‘Hammurabi believed that…often rose in the evening’ to be just a long predicate with the semantic function of attributing some property to the object commonly denoted by ‘Hesperus’ and ‘Phosphorus’. But then the question arises: if attitude reports are not simple predications, what are they? Philosophers have disagreed sharply in their answers. Moreover, their disagreements are intimately connected to a wide range of deep issues about the nature of meaning and reference.
