Final answer:
Hilary Putnam's argument for Semantic Externalism uses the Twin Earth thought experiment to demonstrate that meanings are not determined just by mental states but also by external factors, such as the actual nature of the substances we refer to.
Step-by-step explanation:
Hilary Putnam's First Argument for Semantic Externalism (Twin Earth) is a philosophical concept that challenges traditional views about the mind and language. In the famous thought experiment, Putnam imagines a twin planet Earth (Twin Earth) where all appears the same as on our Earth but the substance called 'water' is not H2O, but rather a chemically distinct substance, XYZ. This substance behaves like water but has a different molecular structure. The inhabitants of Twin Earth call it 'water' as well, yet upon investigation, it is decidedly different.
Putnam's argument challenges the notion that meaning is determined only by what's in our heads and suggests that it is also contingent upon external factors—like the actual chemical structure of substances we are referring to. On Earth, our term 'water' refers to H2O, but on Twin Earth, it would refer to XYZ. Therefore, 'water' has different meanings in different environments despite our internal mental states being the same. This leads to the conclusion that meanings are not solely determined internally and that the external world plays a crucial role in language.