Can you explain the philosophical concept of duality and non-duality as presented in the introduction of Chapter 12?

The beginning of Chapter 12 presents Sadguru Sainath as the embodiment of Truth, Consciousness, and Bliss, and a force that stills the turbulent waves of worldly existence. The text touches upon a core philosophical concept by stating that through the knowledge of non-duality, Sai Baba removes the "obsession with duality." This implies that He helps individuals, even those who are dull-witted, to overcome the perception of separation and conflict that defines a dualistic worldview. This praise highlights His role as a Guru who imparts the ultimate wisdom of unity and oneness, which is the essence of non-duality.