Armstrong's universals are "sparse": not every predicate will have an accompanying property, but only those which are deemed basic by scientific investigation. The ultimate ontology of universals would only be realised with the completion of physical science. Mass would thus be a universal (subject to mass not being discarded by future physicists). Armstrong realises that we will need to refer to and use properties that are not considered universals in his sparse ontology—for instance, being able to refer to something "being a game" (to use the example from Wittgenstein's "Philosophical Investigations"). Armstrong then suggests that a supervenience relation exists between these second order properties and the ontologically authentic universals given to us by physics.