Platonism
Strictly, the philosophy of Plato (c.427-c.347 BC), but the word is often applied to any view which treats a given subject-matter as involving substantial, though abstract, entities (irrespective of Plato’s own view on the topic in question). Such subject-matters have included numbers, propositions, universals (roughly, things named by words ending in ‘-hood’, ‘-ness’, ‘-ty’). Platonism is thus