  Lemmon, in a draft of material intended for his and Dana Scott’s projected book ‘Intensional Logic’, mistakenly credited Geach with the idea that the binary relation ‘may be intuitively thought of as a relation between possible worlds’. In a letter to Scott, written after Lemmon’s death in 1966, Prior remarked: ‘What Geach contributed was not the interpretation of \([U]\) as a relation between worlds (God knows when that started), but the interpretation of \([U]\) as a relation of accessibility’. When Prior said ‘God knows when that started’ he was presumably referring to the idea that the ‘objects’ of the calculus be regarded as possible worlds. He was right to think that the history of this idea is a tangled one. Priority is often assigned to Leibniz, but scholars have traced the idea back to Duns Scotus and William of Ockham (Knuuttila 1993).