Nothing To Fear
The Doctor stops and spins around, crouching down to look up at Christina from the chair. To the question on whether it was an alien, he says, "Possibly. Probably." He bounces to his feet again, and says, "But there's other possibilities that might fit the facts. There were beings on earth longer than humans. Even longer than the Silurians. Beings who are usually invisible, but can sometimes be seen by certain children, whom they violently protect even from the slightest threats like a stern parent. I detected traces of what could be magic — that is, those focused psychic phenomena that humans called magic — which could be from the Fair Folk, or what attracted them to Sam. But either way, how would he know to build a device like that?"
This last question is asked rhetorically, as he almost immediately says, "The clues aren't fitting together yet. We need to see that book. But someone should stay with Sam."