The Master was once a childhood friend of the Doctor. In "The Sound of Drums" (2007), there is a brief clip of the young Master as the Doctor talks about his childhood. When they joined the Time Lord Academy at the age of eight, they were taken to look into the "Untempered Schism". This was a portal through which you could see the time vortex (the tunnel the TARDIS travels through as it travels in time) and see the full power of the Time Lords. He suggests that this is what made the Master go mad.