 At the beginning of "The Trial of a Time Lord",<ref name="The Trial of a Time Lord"></ref> the Sixth Doctor suggests that a number of elder Time Lords were able to use their combined mental energy to summon his TARDIS against his will.Time Lords can survive, but not function properly, without two hearts.<ref name="shakespeare"></ref><ref name = "three"></ref> In "The Shakespeare Code", the Tenth Doctor has only one heart working.He knows this and tries and fails to stand up, until both of them start working again.