Which two Doctors met in " Time Crash " ?
Before that , the only references to past incarnations ( from 1963 to 1996 ) have been in the aforementioned episode " School Reunion " ( in which the Doctor acknowledges having regenerated " half a dozen times " since last seeing Sarah Jane ) and in drawings that the Doctor ( who has temporarily become human to hide from the Family Of Blood ) makes based on dreams of his other life in the 2007 episode " Human Nature " .