 The first constraint binds what is true in some possible world to what one knows a priori. A statement is primarily conceivable if nothing that is knowable a priori is incompatible with the statement being true. As Chalmers explains, that captures a kind of epistemic possibility. Consider again the statement “water is not H2O”. The actual chemical composition of water, H2O, is not knowable a priori. So this fact does not constrain what is true in the possible worlds in which the existence of water is a priori conceivable. Accordingly, “water is not H2O” is primarily conceivable. As Chalmers also puts it, primary conceivability amounts to coherently conceiving the situation described as actual, namely as a way the world might turn out to be.