Differentiation is the ability to maintain your sense of self when your partner is away or when you are not in a primary love relationship. You value contact, but your don't fall apart when you are alone.
We can the understand the difference between differentiation and fusion. Jealousy is a form of emotional fusion. At its most severe, jealousy illustrates our intolerance for boundaries and separateness from those we love. Our desire to possess our partner is inherently frustrated by the immutable fact that we are two fundamentally separate, though interrelated people.
You can see emotional fusion in the mayhem we commit in our relationship, in our inability to separate, to leave well enough alone, when we are on the edge. Our most celebrated media events are often tragic tales involving emotional fusion.
Emotional fusion also fuels the dramas of many people. Don't believe for a moment that "loving too much" is an excuse for physical violence, for emotional fusion increases domestic violence.
As partners you have about the same tolerance for intimacy, although you may express it differently.
Differentiation allows us to set ourselves apart from others and determines how far apart we sit, it also opens the space for true togetherness.
We have promised ourselves paradise through self knowledge, love, sex, and transcendence will be easy once we know ourselves and our partner. But that is often when you need to soothe your own heart and calm your own anxieties to take care of yourself. That is what differentiation offers.
Schnarch, Passionate Marriage