Introduction: Everyone familiar with the current mind-body debate has probably heard about Frank Jackson’s neurophysiologist Mary. So I tell her story very briefly. Mary knows everything there is to know about the neurophysiological basis of human colour vision but she never saw colours herself (she always lived in a black-and-white environment). When Mary is finally released into the beauty of the coloured world, she acquires new knowledge about the world and – more specifically – about the character of the visual experiences of others. This appears clear at first sight. In the ongoing philosophical debate, however, there is no agreement about whether Mary really gains new knowledge and about whether this would, if it were so, represent a problem for physicalism. Those who defend the so-called argument from knowledge (or ‘knowledge argument’) think that it does.