According to the text, what are the prerequisites for understanding the science of Self-knowledge, and why is it so difficult to attain without a Guru?

The provided text from Chapter 20 describes the science of Self-knowledge as a powerful "weapon that cuts through birth and death." However, it is not accessible to everyone. The text explicitly states that only those who are humble and detached are worthy of discussing and understanding it. The difficulty in attaining this knowledge without a Guru is a recurring theme. It is mentioned that the Upanishads, which contain this science, are full of deep meanings and hidden truths. Without the Guru's grace, this "hidden truth does not come to hand," and the path is "difficult at every step." Conversely, for a devotee attached to the Guru's feet, the hidden meaning reveals itself automatically.