One ought too many

[Extract] Normative branches of philosophy take special interest in claims expressed by sentences of the form ‘S ought to φ', where S is some agent and φ-ing is some action (broadly conceived). We'll call sentences satisfying this description agential. These claims raise many important philosophical issues, such as: Which ones are true? What makes them true? How can we know if they are true? What follows from them? Some philosophers have hoped that light would be cast on these issues if we could get clearer on the exact meaning of this word ‘ought’. Once we start looking to language for philosophical clues we should notice that the word ‘ought’ is also employed in different kinds of sentences. It appears in normative sentences with different grammatical forms, like ‘It ought to be that Charlie was born a girl’, and ‘There ought to be less suffering in the world’. For convenience we'll call sentences like these nonagential.1 (It also has a non-normative epistemic use meaning approximately what is probable, as in ‘The sky ought to get dark soon’.) In these other sentences, ‘ought’ appears to represent a propositional operator, O(p)