What is a philosophical question?

There are many ways of understanding the nature of philosophical questions. One may consider their morphology, semantics, relevance, or scope. This article introduces a different approach, based on the kind of informational resources required to answer them. The result is a definition of philosophical questions as questions whose answers are in principle open to informed, rational, and honest disagreement, ultimate but not absolute, closed under further questioning, possibly constrained by empirical and logico-mathematical resources, but requiring noetic resources to be answered. The article concludes with a discussion of some of the consequences of this definition for a conception of philosophy as the study (or "science") of open questions, which uses conceptual design to analyse and answer them.