The latter half of the 20th Century witnessed an ‘intensional revolution’: a great collective effort to analyse notions which are absolutely fundamental to our understanding of the world and of ourselves in terms of a single concept. This was the concept of a possible world: a way things could have been. The idea was used to analyse meaning, information, knowledge, belief, causation, essence, supervenience, conditionality, as well as nomological, metaphysical, and logical necessity.

Possible worlds found applications in logic, metaphysics, semantics, game theory, information theory, artificial intelligence, and the philosophy of mind and cognition. However, possible worlds analyses have been facing numerous problems. The problems all concern hyperintensionality: the need for distinctions more fine-grained than the possible worlds apparatus can easily represent.

One way to address the problems is by introducing impossible worlds, in addition to possible worlds. Impossible worlds are ways things could not have been. In theory, we then get the benefits of the possible worlds approach but without its limitations.

My work on impossibility covers a number of different but related topics:

The metaphysics of impossible worlds

Epistemic logic and logical omniscience

Modelling information