When my family discusses how we should spend a summer holiday, we start from certain common understandings about our preferences. We prefer self-catering accommodation to hotels, and hotels to campsites. We prefer walking and looking at scenery and wildlife to big-city sightseeing and shopping. When it comes to walks, we prefer walks of six miles or so to ones which are much shorter or much longer, and prefer well-marked but uncrowded paths to ones which are either more rugged or more popular. And so on. These common understandings greatly simplify the task of choosing between holiday destinations and activities, by allowing us quickly to eliminate many options. But what does it mean to say that we prefer one thing to another?