" Yes. You see, the whole nature, shape and even the modern blue pigment of the TARDIS is so deeply unfamiliar to the primitive mind that, although the optic nerve registers its presence, the brain cannot decode what it is seeing. The primitive visual cortex is unable to relay information about it consciously to the viewer. In effect, even though her chameleon circuit is still damaged, she’s as good as invisible. She’ll be just fine. "

― Marcus Sedgwick , The Spear of Destiny (Doctor Who 50th Anniversary E-Shorts, #3)