Remember: If "Hesperus" and "Phosphorus" are just different names for the same thing-the planet Venus-how can it be that "Hesperus is Phosphorus" and "Hesperus is Hesperus" differ in meaning?
Additionally: Many philosophers find the notion of sense obscure. The logician Saul Kripke argued that proper names do not have senses at all. In his view the reference of a proper name is not determined by a sense but by a chain of uses of the name that begins with an act of naming. For example, you may use the name "Thales" to refer to a certain preSocratic philosopher even though you don't know anything about him, as long as you acquired the name from someone who used it to refer to Thales.
source30-SecondPhilosophies by Barry Loewer