 To further clarify, for Chalmers expressions have two dimensions of meaning or content: a primary intension and a secondary intension. In the case of sentences describing Kripkean a posteriori necessities like “water is H2O”, these are two different propositions. The primary intension of an expression like, e.g., “water”, picks out a referent at a world considered as actual; namely, roughly based on the descriptive content that a speaker a priori associates with the expression. Thus, at any world we consider, the primary intension of “water” picks out whatever substance satisfies some description such as “the liquid drinkable stuff in lakes and rivers etc.” (or, otherwise put, whatever plays the water role). At any world we consider as actual, “water” will pick out such a substance. Secondary intensions, by contrast, pick out the referent of an expression at a world considered as counterfactual, namely based on the way a speaker evaluates a given expression counterfactually, given how the actual world is. That requires acknowledging that water is H2O. Assuming, as usual, that having a certain chemical composition is essential to being a certain kind of substance, the secondary intension of “water” picks out H2O compounds at all worlds (otherwise put, it picks out what actually plays the water role). At any world we consider as counterfactual, “water” will pick out H2O. Accordingly, sentences like “water is H2O” are secondarily necessary but primarily contingent. For, while it is possible that the liquid drinkable stuff in rivers and lakes etc. is not H2O, it is not possible that water isn’t H2O. Thus, according to Chalmers, ideal positive primary conceivability allows us to access a specific kind of possibility, namely primary possibility, which are sometimes secondarily impossible. While there is a primary possibility in which water isn’t H2O, there is no secondary possibility in which water isn’t H2O.