Sri Aurobindo
Letters of Sri Aurobindo
Volume I - Part 5
Not sex exactly [is present in the psychic being], but what might be called the masculine and feminine principle. It is a difficult question [whether a man can be reborn as a woman or a woman as a man]. There are certain lines the reincarnation follows and so far as my experience goes and general experience goes, one follows usually a single line. But the alteration of sex cannot 
be declared impossible. There may be some who do alternate. The presence of feminine traits in a male does not necessarily indicate a past feminine birth – they may come in the general play of forces and their formations. There are besides qualities common to both sexes. Also a fragment of the psychological personality may have been associated with a birth not one’s own. One can say of a certain person of the past, “That was not myself, but a fragment of my psychological personality was present in him.” Rebirth is a complex affair and not so simple in its mechanism as in the popular idea.