Imagination can't create anything new, can it? It only recycles bits and pieces from the world and reassembles them into visions. So when we think we've escaped the unbearable ordinariness and, well, untruthfulness of our lives, it's really only the same old ordinariness and falseness rearranged into the appearance of novelty and truth. Nothing unknown is knowable.
In this quote from Act One, Scene Seven of Millennium Approaches, Harper is describing to Prior that it ought to be impossible for him to appear in her hallucination, since she should only be able to hallucinate that which she has already experienced. The audience knows the answer—it is no hallucination but a real encounter. But even if it were not, Harper's theory could not possibly be correct since it is contradicted by Prior's very presence. In Part Two, however, the Angel faces a similar problem to the one Harper describes: she and her colleagues cannot create but must rely on God, or on Prior, to invent things for them. Although Harper does not realize it, human beings are more imaginative and thus more powerful than the Angels, providing support for Prior's decision not to obey them.