TIK 3
Sense and Reference
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Reference &mdash the actual item in the world (extension)
Sense &mdash the means for determining the reference (intension)
Frege noticed that we have different reactions to the following sentences:
1. The morning star is the morning star.
2. The morning star is the evening star.
But Venus is both the morning star and the evening star, so "the morning star"
and "the evening star" have the same reference (refer to the same actual item).
But we don't feel that sentence (2) is tautologous. Therefore the meaning of a
sentence involves more than reference, it also involves how one determines
reference.
It is also possible for a NP to have sense without having reference:

The present king of France is bald.
"The present king of France" fails to refer — there is no such actual person. But
sense of it is clear: a person who presently rules the country of France as a
monarch. So we know how to go about determining the reference of "the present
king of France" though that search actually comes up empty.
http://www.ling.udel.edu/idsardi/101/notes/semantics.html