 The notion of a collection is as old as counting, and logical ideas about classes have existed since at least the “tree of Porphyry” (3rd century CE). Thus it becomes difficult to sort out the origins of the concept of set. But sets are neither collections in the everyday sense of this word, nor “classes” in the sense of logicians before the mid-19th century. The key missing element is objecthood — a set is a mathematical object, to be operated upon just like any other object (the set \(\mathbf{N}\) is as much ‘a thing’ as number 3). To clarify this point, Russell employed the useful distinction between a class-as-many (this is the traditional idea) and a class-as-one (or set). 