The idea behind past life regressions is that something that occured in a past life was so tramatizing that it is affecting the current life. In dealing with this issue through hypnotheropy a person will be able to work through what ever lingering problem is stopping them from moving forward.
This theory is based on the idea a person's soul is immortal. That it's engaged in process of reincarnation. According to this theory one can have memories of past life.
Based on reincarnation, it is the idea that we have lived multiple lives, those lives are lessons, and we will be reborn until we transcend (learn the lesson)
In order to move on from this life we must learn why things are happening to us. Believe things will turn out for better of for worse. Its based on Karma and re-birth.
