ABSTRACT
Deontic statements have truth values, so that some inferences that make use of deontic statements are deductively valid. But deontic statements are often used prescriptively, to get people to do certain actions. Therefore, deontic statements can serve as conclusions of valid practical inferences, and people should be able to use new deontic logic (NDL) to show the deductive validity of at least some practical reasoning. Deontic logic, at least as it concerns ought to do statements, is therefore part of the logic of practical decision-making. The chapter considers various factors, whether moral, legal, or prudential, whether relevant or irrelevant, whether justifiable or unjustifiable. Moreover, it has little hesitation in supposing that people sometimes make mistakes in practical reasoning: One person has failed to consider a factor, another has given a factor the wrong weight, and yet another has failed to see the full relevance of a factor.
