In Frege’s view, the same sense cannot determine different references, but philosopher Hilary Putnam (1926-2016) proposed a counterexample (Putnam 1973). Consider a Twin Earth where everything is the same as on Earth, except that what they call water has the chemical composition XYZ instead of H2O. Everything is the same in other respects. On Twin Earth, you and I and everyone else has an identical twin. When my Twin Earthian drinks water, I do it as well, and we have type- identical mental states—they are indiscernible apart from being numerically distinct. Let us suppose neither I nor my twin know the chemical composition of what we call water. We simply think of water as what runs in rivers, what we drink, what we swim in, and so on. However, in Putnam’s view, what my twin means by water is not what I mean by water, even though what is in our heads is type identical.
According to Putnam, this shows that sense does not uniquely determine reference. The idea that we can grasp meaning in a world-independent way is also mistaken. My twin and I grasp the same sense but mean different things. This leads Putnam to conclude that meanings are not in the head. Where are they? In Putnam’s view, we must take the environment into account. Meanings are partly environmentally constituted.