What is the 'causal body' and what is its relationship to ignorance and the soul?

The text from Chapter 50 describes the 'causal body' (karana) as being synonymous with the 'Unmanifest' (Avyakruta). It is characterized as being with the 'reflection of consciousness' and is identified as the very cause of 'pure ignorance.' The source of this causal body is the 'ignorance of the Soul' itself. It is a unique entity, described as being neither with parts nor without parts. The chapter states that the only way to eliminate this causal body is through gaining the 'knowledge of the unity of Brahman and the Soul,' which is the key to dispelling the ignorance it represents.