That's a really funny smell.
It's almost human but not quite, and it's nowhere near hybrid or wolf. A new species, or an unusual witch? He goes check it out.
The Master returns with four more textbooks and shoves them at him.
And it's an elementary school textbook that involves time travel!!!!! Can you change the past? The future? Are stable time loops possible? What can you do?
The past and the future can be freely traveled to and altered, with the exception of fixed points, points in time that can not be reached with Time Travel and have an extremely low elasticity, making them almost impossible to change from other points in the past.
Elasticity is time's resistance to change. It means thats changing details will not necessarily change the general shape of history. The elasticity of a given point in time decreases the closer that point is to a fixed point and increases with the number of time traveler's who have affected that point.
Time travelers retain their memories of their original timeline, even if that timeline changes or is erased. Time Lords are unique in their ability to retain memories of every time line they have experienced. This means that a Time Lord doesn't have to worry about becoming a time orphan, the book reassures the children. Even if time changes so they were technically never born, their family and friends will remember them. It is also why taking any other species time traveling is a very very bad thing, as they could become a time orphan and have no home to go back to.
Time loops are not mentioned.
"What's wrong with you?"
"You can alter timelines. You can alter timelines. Please tell me there is something that does not make this be the same as repeated genocide."
"Besides it not involving killing people?"
"Yes, since killing is only a technicality when they stop existing altogether."
"By that logic you're committing genocide every day, but it doesn't matter since I'm not actively trying to change the future. I could dance around naked and the same people would be alive two hundred years from now."
"- well there's an appealing image," he comments. "Anyway, a different way to put it would be, when you change a timeline do the people who lived in it never get to experience their full lives?"
"Maybe they get different full lives. I mean, I doubt you're going to forget about the alien you found hiding in a tree, but why is this future any less real?"
"I'm not saying it's less real, I'm saying that if someone grows up to be twenty-five and time changes so they're never born that's the same as if they died, but if the whole timeline is fixed sans external influence then it's meaningless to say they lived to any given specific age."
"You could have a kid right now and they'd live to be 25, but you're not. Murderer. When I'm in time, I'm affecting it the same way you are. I'm not using some special time traveler magic to erase people. Right now, there isn't one future."
"...is there only one past?"
"That lead to this particular moment? Yes."
"Are there other pasts leading to other moments that are like this one happening simultaneously?"
"That question makes absolutely no sense, but sure."
"I mean are there other Earths, other Saddes who are vampires and have not met you who have similar circumstances to mine, et cetera."
"Other Earths? Yes. Other Saddes? No clue, but Hitler shows up quite a lot."
"...what a delight. But okay, suppose you kick me out of your fancy spaceship right now, and then go back to thirty years ago and make my parents never meet. What happens to me?"
"How would I find your parents? Okay fine, if I did that and murdered one of them so they wouldn't meet again two days later, you wouldn't be born. I suppose selectively messing with the past to erase one person could be murder, if that's what you're on about. But, if you're going to tell me that existing in this point of time is genocide because I might accidentally stop some people from being born, but existing in the time I was born in isn't even though I could do the exact same thing, I'm going to call you an idiot."
"No, I'm not saying that, I'm saying that if you go back in time and change something that's already happened that does sound a lot like murder. Depending on how it affects stuff. Although apparently the butterfly effects is bullpoop."
"This moment's back in time for all of the possible people living a hundred years from now. If one of them came back and prevented their own timeline from occurring it'd be murder, but it wouldn't be murder if you accidentally prevented that exact same timeline? Fine, I guess I'm a serial killer then. You should run away and pretend you've never met me."
"But if there are multiple possible futures but not multiple possible pasts that's implied, isn't it?"
"No. Look, the past you consider constant is only constant because of where you're standing. From other points in time, the future you consider nebulous is just as constant. You experience time linearly, but your great grandparents are just as real as you are and to them, this might not happen. I could have met you ten years from now and we'd be having the exact same argument, but your 'one' past would be a little bigger."