Just a little taste for you of the kind of stuff I’m reading at the moment. The sauv blanc helps, at least in moderation. 😉

If Q is an explanation-seeking question (e.g. ‘Why did Nero fiddle?’), and q is the indirect form of the question (e.g. ‘The reason that Nero fiddled is that______’), and if a person A is seeking to understand q, and if q_{I} is the answer to q under a specific set of instructions, I (so, for example, it might be ‘Explain why Nero fiddled in terms of his mental state’ or ‘Explain why Nero fiddled in terms of historical factors obtaining in Rome at the time…’ and so on), then:

A understands q

_{I}only if (∃p)(p is an answer to Q that satisfies I, and A knows of p that it is a correct answer to Q, and p is a complete content-giving proposition with respect to Q). (Achinstein, 1983, p. 57)

∃ is the ‘existential quantifier, which means ‘there exists’, so ∃p means ‘there exists a proposition p such that…’

A ‘complete content-giving proposition’ is complex, but basically it means it contains everything relevant and nothing irrelevant to explaining Q.