notes in the margin of error

Dr. Marius: "What is it?"
K9: "Unidentified noetic viral type organism seated in the mind-brain interface, and therefore having no ascertainable mass or structure" (Doctor Who: The Invisible Enemy).

What this means, they explain, is that the virus is only detectable during consciousness, and feeds on intellectual activity. Leela is immune because her "way of looking at things" is instinctual.

It gets better. The Doctor miniaturizes a clone of himself and goes inside his brain in order to locate the virus. They come upon a vast ravine.

Doctor: "This is the gap between one side of my mind and the other."
Leela: "it's dark on the other side!"
Doctor: "Well of course it's dark! That's the gap between logic and imagination! You can't see one side from the other side."
Leela: "But it is there? There is something on the other side?"
Doctor: "This is the mind-brain interface, Leela! At least I think it is. That's the mind (points ahead); that's the brain (points up). Two things, entirely different, but part of the same thing."
Leela: "Like the land and the sea?"
Doctor: "Yes. Sometimes I don't think I quite understand it myself." (She crosses, holding on to his scarf.)

Perhaps the explanatory gap is to be bridged on foot.