room is clean. C1. Therefore, the room is clean or grass is green. However, a similar inference does not seem to be valid for imperatives. Consider: P1. Clean your room! C1. Therefore, clean your room or burn the house down! Ross's paradox highlights the challenge faced by anyone who wants to modify or add to the standard account of validity. The challenge is what we mean by a valid imperative inference. For valid declarative inference, the premises give you a reason to believe the conclusion. One might think that for imperative inference, the premises give you a reason to do as the conclusion says; While Ross's paradox seems to suggest otherwise, its severity has been subject of much debate. The semantics for Deontic logic requires that all obligations in the domain of discourse be fulfilled in an acceptable possible world; The conclusion "It is obligatory to clean your room or burn the house down" does not falsify the premise "It is obligatory to clean your room". In addition, based on the context, it may also be true that "It is obligatory to not burn the house down", in which case any acceptable possible world must have "Your room is cleaned" and "The house is not burnt down" to be both true. Some strands of this debate connect it to Hans Kamp's paradox of free choice, in which disjunction introduction leads to absurd conclusions when applied under the scope of a possibility modal. Mixed inferences The following is an example of a pure imperative inference: P1. Do both of the following: wash the dishes and clean your room! C1. Therefore, clean your room! In this case, all the sentences making up the argument are imperatives. Not all imperative inferences are of this kind. Consider again: P1. Take all the books off the table! P2. Foundations of Arithmetic is on the table. C1. Therefore, take Foundations of Arithmetic off the table! Notice that this argument is composed of both imperatives and declaratives and has an imperative conclusion. Mixed inferences are of special interest to logicians. For instance, Henri Poincaré held that no imperative conclusion can be validly drawn from a set of premises which does not contain at least one imperative. While R.M. Hare held that no declarative conclusion can be validly drawn from a set of premises which cannot validly be drawn from the declaratives among them alone. There is no