 McTaggart’s argument for ontological idealism resists easy summary. In outline, the main moves are as follows. McTaggart first argues that every substance is gunky, that is, every substance is such that it has a proper part that is also a substance. So for each substance, there is an infinite series of substances that are parts of it (Nature of Existence I, chapter XXII). McTaggart holds that a priori reflection reveals that every substance is necessarily gunky, although there might be respects in which some substance is simple. (Were there to be persisting material atoms, there might be entities that lack spatial parts, but they would nonetheless have temporal parts.)