We are standing on the hilly north shore of the Thames estuary, looking out to sea. A man with a stick and bright red waistcoat is walking with his collie dog towards two gaunt ruined towers that stand as a kind of gateway into the glistening panorama of clouds and water beyond. Immediately below, some cows are grazing on a steep slope, their herdsman lolling nearby. Between us and the view seagulls hover, defining the airy space in which the landscape is set.