Extensionality vs Intensionality: A Perspectival Account of Conditional Ought with Definite Descriptions

Abstract

The theme of extensionality in first-order deontic logic has been thoroughly studied in the past, but not in the context of a combination of different types of modalities. An operator is extensional if it allows substitution salva veritate of co-referential terms within its scope and intensional if it does not. It can be argued that one distinctive feature of “ought” (as opposed to the other modalities) is that it is extensional. The question naturally arises as to whether it is possible to combine extensionality and intensionality of different modal operators in the same semantics without creating the deontic collapse. We answer this question within a particular framework, Åqvist’s system F for conditional obligation. We develop in full detail a perspectival account of obligation (and related notions), as was done for Standard Deontic Logic (SDL) by Goble. It is called “perspectival”, because one always evaluates the content of an obligation in one world from the perspective of another one. This requires using some form of cross-world evaluation to handle non-rigid terms like definite descriptions. The proposed framework allows for a more nuanced way of approaching first-order deontic principles.