Theories of personal identity have been divided into three kinds: closed individuality which looks to an ongoing process that begins at birth and ends at death, open individuality for which there is only one 'I' and the apparent distinctions between individuals merely superimposed and virtual, and empty individuality for which personal identity is wholly contained within infinitesimal monadic instants. Closed individuality, which is roughly the common sense view founders on the issue of what it is that 'carries' identity in a world of naturalistic objectivities, and so pursuing this notion to its end leads one to either open or to empty individuality, which are both taken to be philosophically defensible. Both of these seem counterintuitive but are witnessed by certain kinds of non-ordinary experience. Open individuality in particular seems to be close to the Samkhya philosophy which pushes the self-other distinction further and further upstream of experience until all distinctions fall within the objective pole of experience. In each conscious instance however, what is experienced is experienced for a subject and objective reality only gains its incorrigible transcendence from the the consensus experience of distinct subjects. If all subjects are revealed to belong to a single 'I' then transcendence becomes reducible, is seen to be a single object of a single subject and the distinction between these two poles cannot be sustained. (An important intermediate step here is the realisation that consciousness itself falls into the objective pole.) One can glimpse here, through the breakdown of logic, that the inevitable resolution of this path is in non-duality - not-one and not-two, not-both and not-neither.
(Aside: Sartre's phenomenological ontology is also a version of Samkhya, and not only because of the bilingual pun, pour-soi = purusha!)