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 does not arise from the pure self, which is non-dual and complete, nor from insentient matter, which lacks consciousness. Instead, it is the ego that raises this question to itself because it is the only entity that does not know its own origin. The ego typically identifies its existence through sensory experiences, thoughts, and feelings. By asking "Who am I?", the ego is essentially questioning the source of its existence and its current state of being.