stems and spatial patterns are species of the same genus, namely pattern, and that a clear view of spatial patterns throws light on some aspects of the ontological nature of causal systems. In particular, it is argued that all patterns (and systems) depend on a fiat delimitation of something which in itself is a unity without borders. Pattern realism is true. Keywords: Foundations, Kinds of entity, Top-level ontological taxonomies, Patterns. Most top-level ontologies in the history of philosophy reckon in some kind of primary substance (be it things, events, or something else), properties, and relations, and they have some place for space and time. Some other categories like causality and quantity are also almost universal in this sense. Very seldom, however, has pattern got a place high up in any ontological hierarchy. The aim of this paper is to show that patterns really deserve such a place. Of course, patterns are always patterns of something. Therefore, pattern as a category cannot be logically basic, but it is important nonetheless. In some modem ontologies, systems are given a prominent place. In particular, this is true of Mario Bunge's Treatise of Basic Philosophy. The fourth volume of this treatise is given the subtitle Ontology II: A World of Systems, and is wholly dedicated to systems. It opens with the following two sentences: "Every science studies systems of some kind, whether natural (physical, chemical, biological, or social) or artificial (technical). Moreover most sciences study nothing but systems"([ I] p. l ). I agree whole-heartedly. However, in my view, systems make up some special species of the genus pattern. And that fact and some other things are not made clear by Bunge. I will here try to develop further some ideas about patterns and systems which I have put forward earlier (see [2) chapters 4, 6, 7, 14, and 15). Whatever the practical use of ontologies is, and will be, now when information technology revolutionises the world, one of their theoretical uses is to create a bird's eye vie