A Modal Approach to Intentions, Commitments and Obligations:Intention plus Commitment yields Obligation

In this paper we introduce some new operators that make it possible to reason about decisions and commitments to do actions. In our framework, a decision leads to an intention to do an action. The decision in itself does not change the state of the world; a commitment to actually perform the intended action changes the deontic state of the world such that the intended action becomes obligated. Of course, the obligated action may never actually occur. In our semantic structure, we use static (ough-to-be) and dynamic (ought-to-do) obligation operators. The static operator resembles the classical conception of obligation as truth in ideal worlds, except that it takes the current state as well as the past history of the world into account. This is necessary because it allows us to compare the way a state is actually reached with the way we committed ourselves to reach it. We show that some situations that could formerly not be expressed easily in deontic logic can be described in a natural way using the extended logic described in this paper.