Dissociation - which Bing dictionary defines as "the separation of a group of usually connected mental processes such as emotion and understanding from the rest of the mind as a defense mechanism" can be used in BPD; because it's a way of avoiding or not facing difficult feelings, which is obviously a common challenge in BPD. There are different degrees of it; a mild version might be someone who spaces out watching TV and eating junk food for hours. An extreme version would be if someone is being abused and imagines themselves as two people with one watching from above and the other experiencing the abuse. I have never experienced that, but have read about i.