Idealization is a central feature of human thought. We build ideal models in the sciences, our politics is guided by pictures of impossible utopias, and our thinking about the arts and moral life is guided by images of how things might have been. In all these cases we sometimes proceed with a representation of the world that we know is not true or aim at a world we accept we cannot realize. This is the world of the "as if," which the philosopher Hans Vaihinger delineated at the turn of the century, in ways he traced back to Kant. In this book, I aim to explore idealization in aesthetics, ethics, and metaphysics, as well as in the philosophy of mind, of language, of religion, and of the social and natural sciences. No one could be an expert on all of these things, but sometimes in philosophy it helps to stand back and take a broader view. On the way I hope to illuminate many issues, large and small, but there is one over-arching lesson: our best chance of understanding the world must be to have a plurality of ways of thinking about it. This book is about why we need a multitude of pictures of the world. It is a gentle jeremiad against theoretical monism.--