The Doctor shakes his head, "It doesn't surprise me. I've met devils, or those who claim they were, who possessed others partly, I imagine, for that very reason. Had a nasty confrontation with 'em." He remembered being sent down into the the pit on the asteroid all too well.

Brightening up at Donna's question, "Oh that's all hers! She's brilliant! I think she fancies you, really." He probably wasn't suppose to say that, but cultural norms were still a foreign concept to him.