One

Time Lords have two hearts, and humans have only one.

If you were to put all the brilliance in a Time Lord's brain into human grey matter, you'd need a million tiny human brains, more. There's a limit to what can fit in one human brain.

There had only ever been one Time Lord who'd thought of humans as anything more than one more vaguely sentient species.

And now, deep inside the volcano, as Donna Noble places one human hand over his, and takes half the responsibility into her one brilliant human heart, the Doctor knows he's always been right.