   In POM, Russell had postulated two types of composite entities: unities and aggregates (POM, 140f). By a “unity” he meant a complex entity in which the constituent parts are arranged with a definite structure. A proposition was understood to be a unity in this sense.  By an “aggregate”, he meant an entity such as a class whose identity conditions are governed entirely by what members or “parts” it has, and not by any relationships between the parts. By the time of the publication of the first edition of Principia Mathematica in 1910, Russell’s views about both types of composite entities had changed drastically. 