There is no exact equivalent for our concept of interest in ancient Greek and it is not an altogether satisfactory term of art in politics now. The word seems not to have appeared in English political writing until the later sixteenth century, and by the end of the twentieth, had often come narrowly to connote a regard for self and indifference to others. But this does not have to be so, and there is no more satisfactory term. Its explanatory force in English, as in the Greek, lies in what is to someone's advantage, and even if advantage in politics may not always include other people, it will invariably be formed in relation to them. And it is not always simple. ‘What's thy interest in this sad wreck? How came it? What is it? Who art thou?’, the Roman tax-collector asks what he does not know to be the sexually disguised daughter of the king of the Britons who, in the gathering complications of Shakespeare's Cymbeline, has just woken on the headless body of an enemy whom she has mis-identified as a former ally and lover (4.460–1). Thucydides saw that complications arise at every point: in just how clear an interest is, who has it, its nature and how it comes to be formed and informed (or misinformed) as it is; in the differences between the interests people admit and those they might be thought truly to have; in the relations between their interests and those of others; and in how all these things are affected by circumstance, including the passage of time.