Can you explain the philosophical concept of oneness versus duality as presented in Chapter 14?

In Chapter 14, there is a deep philosophical discussion on oneness (non-duality) and duality. The text teaches that the same Divine Essence that exists in one's own body pervades the entire universe. True wisdom lies in recognizing this oneness in all beings. Conversely, the intellect that establishes duality, or sees separation and difference, is described as "pure ignorance." This attachment to manifoldness leads to the continuous cycle of birth and death. The chapter states that through the Guru's teachings, the mind is purified, allowing one to realize their true nature, where everything, from Brahma to a blade of grass, is seen as pervaded by Brahman.