A Brief Overview of Fregean Logic

What is the difference between the following two statements:

1) The morning star is the morning star

2) The morning star is the evening star

As you might know, the morning star and the evening star both refer to the same entity, the planet Venus. One might then say that 1 and 2 are the same sentence since each sentence is an identity statement about the object it is about. One might say the difference is that a different term is being used for the same entity. Consider 3 and 4

3) The current president of the United States

4) George W Bush

Are 3 and 4 the same? Yes, you might answer--assuming the speaker utters the sentence at a time in which "the current US president" means "George W. Bush" and not some future or past US president, actual or possible.

I bring all of these up to introduce a basic pillar of Fregean Semantics. Gottlieb Frege was a logician and an instrumental figure in the development of (what became) Analytic (or Anglo-American) philosophy. More generally, his theory can be appreciated for its intention and/or objective: the logical analysis of language and meaning.

The basic pillar I referred to earlier is the distinction between the sense and the reference of a proposition.

The best way to explain this is to engage the first dilemma again. The problem with 1 and 2 is they both might be only contingently true if we assume the following:

x: it is possible that any speaker, S, does not know that "the morning star" and "the evening star" refer to the planet Venus

Let's assume that someone exists who doesn't know that the two terms refer to one and the same planet. What is the meaning of the two propositions now? Are we to say that 1 and 2 are meaningless for those that don't know that the morning star and the evening star denote the same object? Clearly, this is a harsh and probably mistaken position, since both propositions seem to have a sense about them, That is, both sentences are intelligible and make sense.

Gottlieb Frege had a solution to this problem. Instead of thinking about the meaning of a proposition referentially, we need to take into account the cognitive significance of the propositions in question. When we do that, we realize that the sense, or the mode of presentation of 1 may or may not be distinct from 2 depending on the context of utterance--that is, the conditions under which the proposition is uttered or thought.

So, for example, if Bill, who is a scientist, knows that "the morning star" and "the evening star" both denote one and the same planet, Venus, then 1 and 2 are equivalent in virtue of the fact that they each have the same significance. They are presented the same way to Bill because Bill knows that the terms denote one and only one object.

Alternatively, if Jackson doesn't know that "the morning star" and "the evening star" both denote one and the same planet, it is understandable that he would think that 1 and 2 were distinct: the way in which the proposition expresses itself for him is distinct in each case. For 1, the morning star is the morning star. For 2, the morning star is not the evening star. The basis for this difference is the observed fact that the morning star is seen in the morning and the evening star is seen in the afternoon.

The sense is the mode of presentation of a proposition. The reference is the actual object that the proposition or the term denotes. Here's another example to clarify:

David thinks that chickens are always talkative.\

If David is the one thinking it, the reference of his thought is "chickens always being talkative"--the property or the quality of chickens as talkative creatures. If David isn't the one uttering the proposition, then the reference changes. The reference is now THE THOUGHT THAT--and NOT the thought of a certain property of chickens. The sense is the way in which the sentence is presented to the thinker or utterer.