A model is a complex of a multiple, integrated systems. A person also consists of multiple, integrated systems. What we call identity depends on the web formed by the composite qualities of these integrated and interacting systems. Who we are depends on the model we live.
The model changes when we find its limits. Either we recognize ourselves as having hit the limit (and so become stable and therefore different) or we defy those limits (and grow a different model). We find the limits of our model either because we go looking for them or because we run into them while trying to do something else.
How do we know when we are at the edge of the model and how do we make a choice to change models or retreat within the one we have? We recognize the edge of the model by the impact that edge has on physiology and perception. The edge represents the boundary that divides and connects two elements: inside and outside touch at the edge. This is as disorienting as being turned upside down: we receive simultaneously the information that the two sides are related and the information that they should not be interacting - they should be separate enough that one does not disturb the integrity of the other.
As edges shake us up, they influence integrity - they change what is held together by the model.