One of Chisholm’s most influential critical papers in ethics is “Contrary-to-Duty Imperatives and Deontic Logic”. A number of philosophers had noted the analogy between statements of necessity (e.g., ‘it’s necessary that red things are colored.’) and statements of obligation (e.g., ‘it’s obligatory that promises are kept.’) They also noted the structural similarities between the alethic modalities of necessity, possibility, and impossibility and the deontic modalities of obligation, permission, and prohibition. It’s natural to think that alethic necessity can be understood as truth at all possible worlds. So, by analogy, some suggested systems of deontic logic in which obligation would be understood as truth at all morally perfect worlds. Statements of permission (and prohibition), would be taken to mean respectively truth at some (or no) morally perfect worlds. This generates a straightforward deontic logic.