Philosophers frequently disagree about what is real. Sometimes they disagree so profoundly on this matter for so long a time that they see themselves as members of schools or parties in a conflict, or as part of larger historical trends that play themselves out over and over in the history of ideas. For example, some philosophers tend to argue for all kinds of invisible metaphysical structure in the world – that numbers, properties, causality, free will, etc., are all entities in their own right, somehow existing independently of our grasp or use of them, as real in their own way as the ordinary physical objects we seem constantly to rely on in daily life. We usually call such people ‘realists’. But others see this outlook as a theoretical extravagance, a needless proliferation of ontology. We tag partisans of such views ‘nominalists’.