Why is a Guru considered essential for attaining knowledge, even for someone who is already wise?

Chapter 50 emphasizes that a Guru is indispensable because ignorance manifests from doubt (vikalpa), which can afflict even a wise person, causing them to become prideful. The text uses the powerful analogy of a single drop of fermented gruel ruining a large pot of milk to illustrate how doubt can corrupt knowledge. It even cites the example of a supreme knower like Shuka, who suffered a loss due to doubt. Therefore, the text concludes that doubt cannot be removed without a Guru, whose role is to purify the mind, allowing infinite knowledge and non-dual Samadhi to be attained.