Karana sarira / Ketu (Sookshma Jeev):
Karana sarira or the causal body is the cause or seed of the subtle body and the gross body. It has no other function than being the seed of the subtle and the gross body. It is nirvikalpa rupam, “undifferentiated form”. It originates with avidya, “ignorance” or “nescience” of the real identity of the atman, instead giving birth to the notion of jiva-bhuta.
Swami Sivananda characterizes the causal body as “The beginningless ignorance that is indescribable”. Siddharameshwar Maharaj, the guru of Nisargadatta Maharaj, also describes the causal body as characterized by “emptiness”, “ignorance” and “darkness”. In the search for the “I am”, this is a state where there is nothing to hold on to anymore.
Bengal Institute of Oriental Studies 