How can two people have the same beliefs about water but mean different things by the word water?

In his philosophy of language, Hilary Putnam argued that meanings are not purely mental entities located inside the head. Instead, he proposed semantic externalism, which posits that the meaning of a term is anchored in the external reality to which it refers.

To demonstrate how two people can share identical beliefs yet mean different things by the same word, Putnam devised the Twin Earth thought experiment. In this scenario, an inhabitant of Earth and their exact duplicate on "Twin Earth" share the same mental states, beliefs, and images regarding "water"—both believing it is the clear liquid that fills oceans and falls as rain.

However, the substance on Earth is ${\text{H}}_2\text{O}$, while the substance on Twin Earth is a different chemical compound called XYZ. Because the word water refers to the specific substance in the speaker's environment that they point to, the term carries a different meaning on each planet. Thus, meaning is determined by the external referent rather than by the speaker's internal mental state.1