Reinhardt Grossmann 
Existence of the World [PDF ebook] 
An Introduction to Ontology

Originally published in 1992. The history of Western philosophy can be seen as a battle between those that insist that the "physical universe" exists and those would claim that there is a much larger "world" which contains atemporal and nonspatial things as well. The central part of this book, and the battle, concerns the existence of universals. Starting with the mediaeval definition of the issue found in Porphry and Boethius, the author then considers modern and contemporary versions of the battle. He concludes that what is at stake between naturalists and ontologists is the existence and nature of a number of important categories, like structures, relations, sets, numbers and so on.