 There could be said to be two rather different ways of characterising the philosophical concept of substance. The first is the more generic. The philosophical term ‘substance’ corresponds to the Greek ousia, which means ‘being’, transmitted via the Latin substantia, which means ‘something that stands under or grounds things’. According to the generic sense, therefore, the substances in a given philosophical system are those things that, according to the system, are the foundational or fundamental entities of reality. Thus, for an atomist, atoms are the substances, for they are the basic things from which everything is constructed. In David Hume’s system, impressions and ideas are the substances, for the same reason. In a slightly different way, Forms are Plato’s substances, for everything derives its existence from Forms. In this sense of ‘substance’ any realist philosophical system acknowledges the existence of substances. Probably the only theories which do not would be those forms of logical positivism or pragmatism that treat ontology as a matter of convention. According to such theories, there are no real facts about what is ontologically basic, and so nothing is objectively substance.