Why is a Guru considered essential for removing doubt and achieving true knowledge, even for a wise person?

According to Chapter 50, a Guru is indispensable because ignorance fundamentally manifests from doubt (vikalpa), which cannot be removed without external guidance. The text warns that when doubt infiltrates knowledge, even a very wise person can become prideful and fall into error. It uses a powerful analogy to make this point: just as a single drop of fermented gruel can ruin a large pot of milk, a small amount of doubt can corrupt a vast store of knowledge. Therefore, the Guru's role is critical in helping a disciple first identify and then completely remove this ignorance, which purifies the mind and allows for infinite 'knowledge' and non-dual Samadhi to be attained.