Simple

The only thing you cannot objectify is your own Self.  That is truly the subject - everything else is objective.  The definition of the Self then could be that which is never subject to objectification.  It is never an object.  It has no attributes, no qualities, no content.  It is simply the presence or activity or fact of knowing in any experience.

Then look again - how far is the experience from the knowing of it?  Isn't the tree just "experience of tree" or the "experienced-tree"?  Isn't the sky just "experience of sky" or "experienced-sky"?  In the same way, isn't "experienced-sky" the exact same as the present "knowing" or awareness "of sky"?

Therefore the knowing or experiencing and the experienced world are not-two.  This is the case already and isn't achieved but simply recognized where it was overlooked before.

Then subject and object really have no meaning and are just useful concepts.