"In space and time, and in time and space, a doctor exists. This doctor – The Doctor – is like no others. The Doctor isn't a medical doctor, or any other kind of doctor you can become by studying a degree. No, this doctor is a timelord – the very last."
River smiles. Simon loves digging up old stories from Earth-that-was. They tell of easier, simpler times for the 'verse. No Alliance rule then, no terraforrming planets (at least, not to start), no planetary wars.
The Doctor cannot exist, but she loves the story. "Tell me more," she says, and so Simon does.