The Aparokshanubhuti is one of Shankara’s most pointed shorter works. The title translates as direct or unmediated realisation, and the text holds the seeker to that standard. The Self is not to be reached by inference, by argument, or by gradual approach. It is the immediate fact of awareness in which all knowing already takes place.
Opening
The means by which liberation is accomplished are devotion to the supreme Self and the discriminative inquiry which arises from such devotion. Of these two, devotion is the cause and inquiry is the immediate means.
What is meant by inquiry
Inquiry is the discrimination of the Real from the unreal, the eternal from the transient, the witness from the witnessed. It is the patient looking back into the awareness in which all experience appears, until the assumption that the witness is one of the things witnessed is finally seen through.
[ A representative selection. The full hundred-and-forty-four verses develop Shankara’s account of self-inquiry as the means to direct knowledge of the Self. The complete edition is at the source linked above. ]