Two-dimensionalism

Two-dimensionalism is an approach to semantics in analytic philosophy. It is a theory of how to determine the sense and reference of a word and the truth-value of a sentence. It is intended to resolve the puzzle: How is it possible to discover empirically that a necessary truth is true? Two-dimensionalism provides an analysis of the semantics of words and sentences that makes sense of this possibility. The theory was first developed by Robert Stalnaker, but it has been advocated by numerous philosophers since, including David Chalmers.

Any given sentence, for example, the words,

is taken to express two distinct propositions, often referred to as a primary intension and a secondary intension, which together compose its meaning.

The primary intension of a word or sentence is its sense, i.e., is the idea or method by which we find its referent. The primary intension of "water" might be a description, such as watery stuff. The thing picked out by the primary intension of "water" could have been otherwise. For example, on some other world where the inhabitants take "water" to mean watery stuff, but, where the chemical make-up of watery stuff is not H₂O, it is not the case that water is H₂O for that world.

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, which 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 is H₂O" is true in every world.

...
Wikipedia