“I walked away from the Last Great Time War. I marked the passing of the Time Lords. I saw the birth of the universe and I watched as time ran out, moment by moment, until nothing remained. No time. No space. Just me. I’ve walked into universes where the laws of physics were devised by the mind of a madman. I’ve watched universes freeze and creations burn. I’ve seen things you wouldn’t believe. I’ve lost things you’ll never understand. And I know things. Secrets that must never be told. Knowledge that must never be spoken.”
Emphasis mine; I’d just watched the Third Doctor serial “The Three Doctors” before watching this episode and was delighted to hear it referenced. If you want a heavy dose of Time Lord history, check it out.