Frege on Meaning
Frege makes a distinction between sense and reference. He believes that singular terms—such as those for planets, cities, people, or other things, including abstract entities—have both sense and reference. The referring expression “the evening star” refers to a specific star in a specific sense. Frege distinguishes between sense and reference because we can refer to the same referent using different senses. We can refer to Venus as the evening star or the morning star. What is meant depends on the sense used.
Fregean senses exist independently of us in an abstract realm, like Plato’s world of forms. Frege thinks an adequate theory of language cannot have subjective senses. In support of Frege’s reasoning, consider how we can use referring expressions and mean the same things without sense-based objectivity. In Frege’s view, we need objective, mind-independent senses to do this. Senses cannot be psychological entities, because human psychology is personal and subject to evolution and change.
Frege is thinking like the logician he is—there is no room for “psychologism” in the philosophy of language.
We understand the referring expression “the Evening star” through a mental state capturing its mind-independent sense that determines, in a specific way, the reference. Meaning is a matter of sense and reference. We can contrast this with Russell’s theory of meaning, which eliminates mediating Fregean senses—the meaning of a referring expression is simply its reference. In this view, propositional contents of thoughts and sentences are individuated by objects and their properties.
Russell’s account of meaning in terms of objects in the world opened up for the development of externalism as a philosophical perspective.
-  See “On Sense and Reference” in Frege (1960). See also Richard Mendelsohn’s discussion ofFrege’s view of sense and reference where he points to further sources from Frege’s writings, alongwith his own analysis in Chap. 3 of Mendelsohn (2005).
-  Russell outlines his theory in Russell (1993, p. 167).