 When combined with the conception of a sentence’s content as the set of worlds where it is true, the distinction between the two concepts of truth in a world yields a distinction between two different propositions expressed by a sentence. The first of these propositions is the function that assigns the true to a world \(w\) just in case the sentence is true in \(w\) considered as actual, while the second proposition is the function that assigns the true to a world \(w\) just in case the sentence is true in \(w\) considered as counterfactual. Jackson calls the former proposition the sentence’s ‘A-intension’ (for ‘actual’) and the latter its ‘C-intension’ (for ‘counterfactual’), while Chalmers calls the former the ‘primary intension’ and the latter the ‘secondary intension.’ The distinction between the two propositions expressed by a sentence yields a distinction between two notions of sentential necessity: primary necessity, which applies to sentences with necessary primary intensions, and secondary necessity, which applies to sentences with necessary secondary intensions. If a sentence has primary necessity, then that fact, and a fortiori the fact that the sentence is true, can be read off its two-dimensional intension. Therefore, if we know the two-dimensional intension, then that is enough to know that the sentence is true. No further empirical evidence is required. That motivates the thought that the notion of primary necessity captures the idea of a prioricity or epistemic necessity. The notion of secondary necessity, on the other hand, may be taken to capture the Kripkean idea of metaphysical necessity.