Acharya Prashant explains that self-inquiry, specifically the question 'Who am I?', is the foundation of all spirituality and Vedanta. He clarifies that this question is not asked by the pure self, which is non-dual and all-knowing, nor by insentient matter, which lacks consciousness. Instead, it is the ego that raises the question to itself because it is the only entity that exists in a state of not knowing. The ego identifies its existence through sensory experiences, thoughts, and feelings; therefore, asking 'Who am I?' is equivalent to asking where one's existence, thoughts, and feelings originate from.