Immediately before incarnation a very important event occurs, parallel to the event which follows the moment of death. Just as immediately after death the whole memory of a man’s past life appears like a tableau before his soul, so is a kind of preview of the coming life given to the soul immediately before it incarnates. Not all the details are seen, but the circumstances of the coming life are made evident in broad outline. This is of the utmost importance. It may happen that a person who went through a great deal of suffering and hardship in his previous life receives a shock from the glimpse of the new circumstances and destiny now in prospect, and holds back the soul from complete incarnation. Only a part of the soul then enters the body, and this will result in the birth of an epileptic or an idiot.
Source: Rudolf Steiner – GA 95 – At the Gates of Spiritual Science: Lecture five: Human tasks in the higher worlds – Stuttgart, 26th August 1906
Translated by E. H. Goddard & Charles Davy