existing in your head, and existing are two entirely different things. I'm talking existence as in it existence as a human being, or as an animal with a brain in its head. We cannot be 100% sure that other people and living things exist, or that they think, we can only be sure that we exist because we are thinking about it. things existing as ideas in your head is different though. Using that side of philosophy you can say that dragons exist, because you can picture them in your head.

but it exists in your head, so it technically exists. lol
it's hard to explain.

*shakes head* No it doesn't. Even if you put yourself in a position of omnipotence, you can't be sure everything around you exists. Like I've said it can all be in your head, which it technically is. So even if you think it exists, doesn't mean it does, you can only be sure of your own existence, since your able to think about it, and you couldn't do that if you didn't exist.

It makes sense if you see yourself as an all powerful god. And everything around you is a part of you. So, since you exist, cause you think, everything else exist too, cause everything is you. lol

I know that I'm just saying your interpretation makes no sense. I can't know for sure you exist, so just because I think doesn't mean you exist. You exist because (presumably) you think.

ye, I know.
but I was replying to your title. lol

It's from the french philosopher Renee Decartes. It stems from the fact, that the only truth you can know for sure is that you exist, because you are thinking. You can't know if others around you are thinking, and if they tell you that they are it could just be your own mind playing tricks on you. So because I am thinking I know that I exist.

you think therefore you are. but what makes you believe that all you see isn't a part of you?
you think therefore I am.
(that's how I interpret that lol)