Central to Armstrong's philosophy is the idea of states of affairs ("facts" in Russell's terminology): in "Sketch for a Systematic Metaphysics", Armstrong claims that states of affairs are ""the" fundamental structures in reality". A state of affairs roughly put is an instantiation of a particular and a universal: a state of affairs might be that a particular atom exists, instantiating a universal (say, that it is of a particular element, if chemical elements are ultimately accepted as part of Armstrong's universals). The particulars in Armstrong's ontology must have at least one universal—just as he rejects uninstantiated universals, he also rejects "unpropertied particulars".