Twin Earth

The scenario

Twin Earth is a planet exactly like Earth in every observable way. The one exception: what Twin Earthlings call "water" is not H2O but a different compound, XYZ, which looks, tastes, flows, and freezes just like water.

In 1750, before chemistry, an Earthling and their molecule-for-molecule identical Twin Earth counterpart are in precisely the same mental state. Both believe "water quenches thirst," both drink it, both call it water. Yet when the Earthling says "water," they refer to H2O. When the Twin Earthling says "water," they refer to XYZ. Same mental state. Different meaning.

Semantic externalism: meaning ain't in the head

Putnam's conclusion is blunt: semantic externalism holds that the meaning of natural-kind terms is not determined by what's going on inside the speaker's mind. It's determined partly by the actual substance in the environment.

This runs against the Cartesian picture, where the mind contains everything needed to fix meaning. Putnam's slogan became a rallying point: "Cut the pie any way you like, 'meanings' just ain't in the head." What you refer to depends on what you're actually causally connected to, not on how things appear to you.

Narrow versus wide content

The Twin Earth case forced a distinction philosophers now call narrow content versus wide content. Narrow content is whatever is shared between you and your twin, the purely internal, substrate-level information. Wide content is meaning as it actually attaches to the world.

Critics of Putnam argue that we should just say the two people have different concepts of water, and that their shared narrow content is what really matters for explaining their behavior. Putnam's reply is that this redescription is fine, but it doesn't save the view that meanings are in the head. It just relabels the problem. The word "water" in English refers to H2O, full stop, regardless of what any individual speaker has stored internally.