Why is a Guru considered essential for removing doubt and attaining knowledge?

Chapter 50 emphasizes that a Guru is indispensable because ignorance often manifests from doubt, or 'vikalpa'. The text states that this doubt cannot be removed without a Guru's guidance. It provides a powerful analogy, explaining that just as a single drop of fermented gruel can ruin a large pot of milk, doubt can corrupt knowledge and make even a wise person prideful. Therefore, a Guru is needed to help an individual first understand and then remove this ignorance, which purifies the mind and allows for the manifestation of infinite knowledge and non-dual Samadhi.