The "secondary intension" of "water" is whatever thing "water" happens to pick out in "this" world, whatever that world happens to be. So if we assign "water" the primary intension "watery stuff" then the secondary intension of "water" is H2O, since H2O is "watery stuff" in this world. The secondary intension of "water" in our world is H2O, and is H2O in every world because unlike "watery stuff" it is impossible for H2O to be other than H2O. When considered according to its secondary intension, water means H2O in every world. Via this secondary intension, Chalmers proposes a way simultaneously to explain the necessity of the identity statement and to preserve the role of intension/sense in determining the reference.