By Lifecycle Stage

The early research papers on SPT suggested that the various ISPs are parts of the successful stages in a general systems lifecycle. There is, of course, no such thing as a general system. All systems are particular instances. But if you compare across the lifetime of many different instantiations, you begin to see a regular series of lifecycle stages and so can cluster the ISPs by reason of which stage they do or do not participate in.

Here is one of the early diagrams of the proposed general systems lifecycle: