Tradition describes the Bhagavad-gita as a treasure chest. The middle chapters (7-12) are where the treasure is kept. This part of Bhagavad-gita is its essence, hidden and specially protected by the Lord. It discusses the nature and activities of pure devotional service. These chapters contain the prayojana tattva, or the goal of bhakti. Bhakti-yoga is the essence of the Gita: "Yoga actually means bhakti-yoga; all other yogas are progressions toward the destination of bhakti-yoga." (Srila Prabhupada, purport to BG 6:47).