Full Idea
The Core Thesis for rationalist 2D semantics is that for any sentence S, S is apriori iff S has a necessary 1-intension. (That is, there is no possible way the world might be that, if it actually obtained, would make S false).
Gist of Idea
A sentence is a priori if no possible way the world might actually be could make it false
Source
David J.Chalmers (Epistemic Two-Dimensional Semantics [2004], p.165), quoted by Laura Schroeter - Two-Dimensional Semantics 2.3.2
Book Reference
'Stanford Online Encyclopaedia of Philosophy', ed/tr. Stanford University [plato.stanford.edu], p.24
A Reaction
[The parenthesis is by Schroeter] A '1-intension' is defined by a diagonal on a 2D semantic matrix. Chalmers defends conceivability as the guide to possibility. This is a very traditional view of the a priori, expressed in modern terms.
Related Ideas
Idea 14710 Necessity is always knowable a priori, and what is known a priori is always necessary [Kant, by Schroeter]
Idea 14704 2D semantics gives us apriori knowledge of our own meanings [Schroeter]