Problems of Intensionality

I have been assuming, as I said, that if we replace one co-referential term by another in a sentence, we should get a true sentence if the original sentence was true, and a false sentence if the original sen­tence was false.

Consider the two sentences “I believe that the Morning Star is Venus” and “I believe that the Evening Star is Venus.” As we have seen, one of these could be true and the other false. Yet the one sen­tence is produced from the other by substituting co-referential expressions. We might conclude that it is just wrong to suppose that substitution of co-referring expressions preserves truth value.

What Frege argued, however, was that “one can only justifiably conclude. .. that ‘the Morning Star' does not always refer to the planet Venus.” If, in the sentence “I believe that the Morning Star is Venus” the name “the Morning Star” does not refer to Venus, then, of course, it does not count as a counterexample to the com­positionality thesis for reference. But this reply should only satisfy us if we have an explanation both of when it does not refer to Venus and why it does not. I will try to offer such an explanation at the end of this section and in the next. Before I do that, let me describe the way Frege set about solving this problem.

In the sentence

F: The Morning Star is the Evening Star

which we considered earlier, substitution of co-referential expres­sions, as we saw, preserves truth value.

F1: --------- is the Evening Star

will produce a sentence with the same truth value as F, provided we substitute into the blank a word, such as “Venus,” that has the same reference as “the Morning Star.” An open sentence like this, which produces a sentence with the same truth value whenever we substi­tute an expression with the same reference for the blank, is called an “extensional context.” (Remember, the reference of a predicate was called its extension.)

On the other hand, the open sentence

I believe that-------- is Venus

size=2 color=black face="Times New Roman">is not an extensional context, as we have seen. If we want to provide terms whose substitution into this blank will preserve truth value, they must be terms with the same sense. Since, as I have said, the sense of a predicate is sometimes called its intension, these are called “intensional contexts.” Frege's solution to the problems raised for his basic theory by intensional contexts was very simple. He proposed that in intensional contexts, words and phrases referred not to their normal references but to their senses.

Though this is a very simple solution, it is also rather hard to get a grip on. It follows from this theory, after all, that “the Morning Star” in

I believe that the Morning Star is Venus refers to the sense of “the Morning Star.” So the sense of “the Morning Star” in this sentence is the mode of presentation of the sense that “the Morning Star” has in extensional contexts. It is the sense of a sense.

Put this way, as I say, Frege's proposal is not very easily under­stood; but we can put Frege's theory in another way, which makes it easier to grasp what he is getting at.

In fact, Frege can offer a general explanation of why “I believe that           is Venus” should create intensional contexts. The effect

of interchanging co-referential terms in the blank here is equivalent to interchanging co-referential sentences in the blank of the open sentence “I believe that       .” According to Frege, the content of

a sentence, the thought it expresses, is its sense. Two sentences with different contents express different beliefs. It is natural, therefore, that interchanging sentences with the same reference but different sense in the context “I believe that-------------------------- ” will sometimes lead us

from truth to falsehood.

Many intensional contexts that involve the attitudes of people to propositions can be explained in this way. People's attitudes to them depend on the thought and not simply on whether it is true. Thus, “I doubt that-------------------------,” “I hope that----------,” “I fear that----------,” “I

know that---------,” “I suppose that----------,” and so on are all inten­

sional contexts for this reason. These sorts of expressions are the names of what are called “sentential attitudes” or “propositional attitudes,” because what fills the blank is a sentence, which expresses a proposition.

Unfortunately, however, not all intensional contexts involve sen­tential attitudes. If, for example, we replace the sentence “It is or is not raining” in “It is necessary that it is or is not raining” with a sen­tence with the same reference—that is, the same truth value—we will not always get a sentence that is also true. Thus “I like celery” is true, but it is not necessarily true. So we must see, now, if we can explain why “It is necessary that—” creates intensional contexts.

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