Validity of Practical Inference

The paper develops a dynamic framework suitable for analysis of practical reasoning. The application of functional approach in dynamic semantics con...rms the results obtained in philosophical analyses of practical inference, regarding, in particular, defeasibility, undetachability and distinctive quality of conclusion. Contradictory results of proposed tests of validity and con‡icting intuitions on valid forms of practical inference are reconciled. The paper is divided in four parts. In Introduction the notion of practical inference is de...ned. The second part displays the syntax and semantics of language of practical F iat=Est logic. The third part gives extensive elaboration of reasons for proposed formal semantical solutions. In the fourth part, the notion of validity of practical inference is de...ned and the decision procedure is given. 1 Practical inference Aristotle discovered that practical inference is a type of inference di¤erent from the theoretical one. Although he did not give an explicit account on the nature of ’practical syllogism’, its attributes are implicitly de...ned. The ...rst attribute seems obvious: in practical inference at least one premise sets a goal. The existence of a goal-sentence in the premise set is the minimal condition to be included in de...niens of practical inference, and, it may be reasonably argued, a su¢cient one. The clear way of describing the process of (instrumental) practical reasoning in Aristotle’s Nichomahean Ethics (1112b, 11-14) deserves our attention: We deliberate not about ends but about means. For a doctor does not deliberate whether he shall heal, nor an orator whether he shall persuade, nor a statesman whether he shall produce law and order, nor does any one deliberate about his end. They assume the end and consider how and by what means it is to be attained; and if it seems to be produced by several means they consider by which it is most easily and best produced, while if it is achieved by one only they consider how it will be achieved by this and by what means this will be achieved till they come to the ...rst cause, which in the order of discovery is last...And if we come on an impossible, we give up the

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