Why did Hilary Putnam say that meanings are not only in the head?

Hilary Putnam famously asserted that "meanings just ain’t in the head!" to challenge the traditional view that meanings are purely mental entities, such as beliefs, mental images, or conceptual constructs.1

In his classic 1975 paper, The Meaning of "Meaning," Putnam argued that meaning is instead anchored in external reality through a position known as semantic externalism. He identified two primary reasons for this:1

• Reference and Causal Chains: The meaning of a term is primarily fixed by its reference—the actual entity in the world to which it points. This connection is established through causal chains of usage and social practices (like pointing or weighing) rather than by internal mental states.1
• The Twin Earth Thought Experiment: Putnam used this experiment to demonstrate that two people can have identical mental lives—the same beliefs and images—yet mean different things by the same word. For instance, if "water" on Earth refers to ${\text{H}}_2\text{O}$ and "water" on a duplicate planet refers to a different substance ($\text{XYZ}$), the meanings differ because the external referents differ, despite the speakers' identical internal states.1

Consequently, Putnam argued that meaning is context-dependent and inextricably linked to the external world.12