Why is a Guru considered essential for removing doubt, even for a knowledgeable person?

According to Chapter 50, a Guru is indispensable because ignorance can manifest from doubt, and this doubt cannot be removed without a Guru's guidance. The text warns that when doubt enters knowledge, even a wise person can become prideful. It provides the example of a supreme knower like Shuka who suffered a loss due to 'vikalpa' or doubt. Using the analogy of a drop of fermented gruel ruining a large pot of milk, the chapter illustrates how doubt can corrupt knowledge. Therefore, a Guru is needed to help remove this ignorance, purify the mind, and allow infinite knowledge to manifest for the attainment of non-dual Samadhi.