Abstract

Following Richard M. Hare1 I think that we use practical sentences as decision criteria. We understand their meaning if we know what decision to take according to them. But it is not clear, how exactly decision criteria are related to decisions and how they function as criteria. To fully understand this role, we need a formal semantics of practical sentences. For this I have to introduce a formal language and give an interpretation of it. This language has to be constructed in such a way that a translation into ordinary language is always possible in principle. Thus, we make sure that our semantics and logic will have an impact on the solution of concrete practical problems. According to this program I will first introduce the formal language ‘LP1’. To give an interpretation of it I will then clarify what a decision is and show how practical sentences function as decision criteria. On this basis I give an interpretation of the primitive two-place operator ‘PT p,q’ and the one-place operator ‘VTp’. I further argue that we make meta-decisions concerning the application of first-order decision-criteria. This allows me to introduce a new concept of practical validity, which differs radically from the concept of truth. Using this concept, I then give an interpretation of the deontic operators ‘OTp’, ‘FTp’, ‘ATp’ and ‘ITp’. The concept of practical validity makes it also possible to introduce practical logical connectors and mixed logical connectors on the basis of practical or mixed value tables. These connectors are used – among others – in bridge-principles, which play an important role in ethical and juridical theories. Finally, I shortly explain the semantics of the main kinds of practical sentences, i.e. value judgments, imperatives, norms and intentions, and I argue that we need a deontic logic in order to use practical sentences in a correct way.