Practical Inference—A Formal Analysis

Most engineering reasoning in practice is about how to achieve some predetermined end. Despite its paramount importance, this form of reasoning has hardly been investigated in the literature.a The aim of this paper is therefore to explore the question to what extent technical norms can be said to have a truth-value, and under what conditions practical inferences are deductively valid. We take technical norms to be sentences of the form ‘If you want A, and you are in a situation B, then you ought to do X’. Von Wright’s standard example of making a hut habitable is our paradigm for practical inferences, where an obligation to act is deduced from an intention to realize an end, and an empirical constraint on how this end can be achieved. Our instrument of analysis is dynamic logic (PDL), since actions are aimed at changing the world. PDL already suffices to provide truth-conditions for technical norms. To accommodate the obligation in practical inferences we draw on John Jules Meyer’s deontic version of PDL. By paraphrasing ‘person P wants’ with ‘person P imposes an obligation on herself,’ we can give a plausible definition of the validity of practical inferences. In the discussion section, we address the issues of the reliability instead of truth-value of technical norms, and of the defeasibility of practical inferences as they occur in engineering practice.