The Abstract and the Concrete presents nine essays in ontology by Peter van Inwagen. Three of the essays concern topics in meta-ontology: the idea of multiple modes of being; Carnap's idea that the questions of "ontology," insofar as they are meaningful at all, are questions about which linguistic frameworks it is expedient to employ; the concept of one object's being metaphysically more fundamental that another. Three of the essays concern various topics that pertain to the author's "lower-case" or "lightweight" platonism. (According to lightweight platonism, there are attributes-necessarily existent universals. These attributes are not constituents of substances, they cannot enter into causal relations, and it is false that an F object is F in virtue of instantiating the attribute of being F.) The remaining three essays examine proposed answers to particular ontological questions: the question of the validity of mathematical fictionalism; the question whether it is analytic that at any place at which some xs are arranged chairwise, there is there a chair; the question of what it means to say that colour is an illusion, and whether (in the sense determined) colour is an illusion.