In rare cases, in spite of the resistance offered by the brain, some memories of past lives leak into the present life in the form of dreams that are completely unexplainable in terms of the present life. An individual may see persons in his dreams whom he has never seen in his present life. It often happens that those who appeared in the dreams were persons whom he had met in his past lives. But of course such dreams, when they are of the ordinary type, cannot be treated as a memory of past lives. They merely indicate that the imagination at work in the dreams was influenced by information taken from past lives. The real memory of past lives is clear, steady and sure like the memory of the present life. When it comes to an individual, he no longer has any doubt about his having existed in several previous lives along with many other individuals. Just as he cannot doubt his own past life in the present incarnation, he cannot doubt his life in past incarnations.
-Discourses 7th Ed, p314