Wimsatt (1997) contrasts mechanistic organization with aggregation, a distinction that mechanists have used to articulate how the parts of a mechanism are organized together to form a whole (see Craver 2001b). Aggregate properties are properties of wholes that are simple sums of the properties of their parts. In aggregates, the parts can be rearranged and intersubstituted for one another without changing the property or behavior of the whole, the whole can be taken apart and put back together without disrupting the property or behavior of the whole, and the property of the whole changes only linearly with the addition and removal of parts. These features of aggregates hold because organization is irrelevant to the property of the whole. Wimsatt thus conceives organization as non-aggregativity. He also describes it as a mechanistic form of emergence (see Section 4.2 below).