Tomoyuki Yamada

Although it seems intuitively clear that acts of requesting are different from acts of commanding, it is not very easy to sate their differences precisely in dynamic terms. In this paper we show that it becomes possible to characterize, at least partially, the effects of acts of requesting and compare them with the effects of acts of commanding by combining dynamified deontic logic with epistemic logic. One interesting result is the following: each act of requesting is appropriately differentiated from an act of commanding with the same content, but for each act of requesting, there is another act of commanding with much more complex content which updates models in exactly the same way as it does. We will also consider an application of our characterization of acts of requesting to acts of asking yes-no questions. It yields a straightforward formalization of the view of acts of asking questions as requests for information.

In this paper, illocutionary acts of commanding will be differentiated from perlocutionary acts that affect preferences of addressees in a new dynamic logic which combines the preference upgrade introduced in DEUL (dynamic epistemic upgrade logic) by van Benthem and Liu with the deontic update introduced in ECL II (eliminative command logic II) by Yamada. The resulting logic will incorporate J. L. Austin’s distinction between illocutionary acts as acts having mere conventional effects and perlocutionary acts as acts having real effects upon attitudes and actions of agents, and help us understand why saying so can make it so in explicit performative utterances. We will also discuss how acts of commanding give rise to so-called “deontic dilemmas” and how we can accommodate most deontic dilemmas without triggering so-called “deontic explosion”.

If the notion of speech acts is to be taken seriously, it must be possible to treat speech acts as acts. The development of systems of DEL (dynamic epistemic logic) in the last two decades suggests an interesting possibility. These systems are developed on the basis of static epistemic logics by introducing model updating operations to interpret various kinds of speech acts including public announcements as well as private information transmissions as what update epistemic states of agents involved. The methods used in developing DEL can be used to develop logics that deal with a much wider variety of speech acts. For example, in ECL (Eliminative Command Logic) of Yamada (2007a) and ECL II of Yamada (2007b), similar model updating operations are introduced tointerpret acts of commanding as what update deontic aspects of the situations in which agents are involved. In Yamada (2008a), ECL II is further extended so as to model acts of promising along with acts of commanding. Moreover, in Yamada (2008b), ECL II is combined with a modified version of DEUL (dynamic epistemic upgrade logic) introduced in van Benthem & Liu (2007). In the resulting logic DDPL (dynamic deontic preference logic), illocutionary acts of commanding are differentiated from preference upgrading perlocutionary acts. The development of these logics suggests a recipe for developing logics that deal with various specific speech acts: first, carefully identify the aspects affected by the speech acts you want to study; second, find the modal logic that characterizes the aspects in question; and finally, add dynamic modalities that stand for the types of the speech acts being studied and define model updating operation that interprets these speech acts as what update the very aspects.