When lovers commune, they learn what cannot be expressed or known in other ways. Or so it is plausibly said. Donne's poem ‘The Extasie’ tells how

A behaviourist would have had a thin time of it. Yet negotiate they did, by a process needing a special sort of Verstehen. The sharpest observer cannot understand, unless he were

I do not doubt that there is a process by which not only lovers empathise. Indeed I would gladly present an epistemology for it, if I could see my way to a pure science of ecstasy. But for present purposes nothing less would do. If the insights can be expressed and known in other ways, then empathy becomes, in principle, merely a short cut. To put it more sonorously, where the usual canons of validation apply, Verstehen is only a heuristic device. The skills of anthropologists in the field, art critics, personnel managers and participant observers in general are not our concern and this chapter is not about empathy.

It is about knowledge of the rational thing to do or the a priori understanding of necessities to which rational action is subject. The crux will be that rational action is a skill, needing knowledge and power. When it is exercised, the actor could have done otherwise but could not have done better. That x was the rational thing to do is, if true, a necessary truth, knowable both a posteriori and a priori. Irrational action is a failure to find the best move and its explanation depends on knowing what the best move would have been. These cryptic ideas will first be deployed with the aid of chess and Kriegspiel, where the distinction between a posteriori and a priori comes out instructively and the skill depends on anticipating an as yet undiscovered theory of the game.