As put forward by Richard C. Schwartz in the book Internal Family Systems Therapy (1995, Guilford Press), internal family system theory describes everyone -- multiple or not -- as having internal "parts" which may become "anthropomorphized". Schwartz disagrees with the concept of anthropomorphization, pointing out that it entails endowing with human attributes something which was not human to begin with. Over time he has come to see that what may look like "parts" are distinct and separate beings sharing the body.
In Schwartz's view of multiplicity, it matters less where the internal entities originate; the important thing is that they are now sharing a home and their best course of action is to get along as a family -- a functional family -- rather than infighting and competing for resources and declaring individual independence.
Before one reacts negatively to the idea that persons in multiple systems cannot be individuals in their own right, consider what Schwartz means by individual independence. He goes into some detail about the myth of independent existence, reminding us that all human beings are interdependent creatures and his view that it's best to reflect this internally as well as externally. (Even self-sufficient persons who live independently of the dominant power structure are still interdependent with the earth -- they'll be getting their food and shelter from natural materials -- and with animals if they kill any for food, clothing, tools etc.)
Most people are societal creatures. In technologically advanced cultures, we depend on people to provide services for us (electricity, apartment rental, transportation, etc.) In small-scale cultures there is even more interdependence; hunter-gatherers are expected to provide food not only for themselves but for the rest of the group. The natural cycle of contribution involves individuals contributing a resource to society, and other individuals dependent on or drawing on that resource, until it's their turn to contribute. This interdependence allows us to free up time from work, giving us time to think, dream, create stories, and so on, as well as specialization of skills or tasks.
Schwartz proposes that multiples can and should run their groups as societies. He assumes that each person in the group will have a skill, talent, interest or inclination toward something and should work on developing it. Each will have their strengths, weaknesses, skills, talents, interests, etc. Schwartz assumes that most multiples try to pull the shared body in all directions at once -- he obviously hasn't met any who have developed a cooperative system without lengthy, intensive therapy -- and proposes the internal family model to regulate matters. Once again, a singlet is telling us how we should run our households, but his structural ideas are not too dissimilar from some operating systems independently created by multiples outside of therapy.
Regarding one's group as an internal family necessitates work on communication, on getting to know each other, on finding out what people in the family need and how that can be provided in a way that doesn't risk the family or any of the individuals in the family, etc.
Sentence structure, grammar and spelling changes, but kept to the essential of the original review. Now I have to finish reading this book too. Astraea^Bluejay