 Fourth, McTaggart distinguishes between compound substances and groups. A compound substance is a substance that has other substances as proper parts. On McTaggart’s view, every substance is a compound substance. A group also has substances as proper parts, but it not merely a compound substance. Rather, it is something more akin to a set or a collection. However, McTaggart explicitly claims that groups are not classes because a class is “determined by a class-concept, while a group is determined by a denotation.” (See page 276 of “An Ontological Idealism” in his Philosophical Studies, as well as chapter XV of the first volume of the Nature of Existence, wherein he claims that classes are determined by properties.)