Donna Noble:  But I 'm history , too . You saved me in _ 2008 _ , saved all of us . Why is that different ?
The Doctor: Some things are fixed , some things are in flux . Pompeii is fixed .
Donna Noble: How do you know which is which ?
The Doctor: Because that 's how I see the universe . Every waking second , I can see what is , what was , what could be , what must not ... That 's the burden of the Timelord , Donna . And I 'm the only one left .