तदहं नात्र सन्देहो विचारः सोsयमीदृशः।।
As I am also the One, the Subtle,the Knower [the Supreme Knowerwho is ever present in all ourperceptions as consciousness,and who perceives even the ego],the Witness, the Ever-Existent andthe Unchanging, so there is no doubtthat I am “That” [i.e. Brahman].Such is this enquiry.
Adi Sankara[Aparokshanubhuti]

When I say, “I know that I exist,” the “I”of the clause ‘that I exist’ forms a partof the predicate and as such it cannotbe the same “I” which is the subject.This predictive ‘I’ is the ego, the object.The subjective ‘I’ is the Supreme Knower.
I, the ego, when stripped of all its limitingadjuncts, such as the body and the like,becomes one with “That,” the SupremeEgo, i.e., Brahman. In fact, it is alwaysBrahman; Its limitation being but thecreation of ignorance.
Swami Vimuktananda