Not necessarily true. It can be in your head without you being conscious of it, like a dream. You mind created the rules for the dream, and you followed it. For all you know, you could be in a coma right now, or be on your deathbed, having a dream that is out of your control. There are many more outrageous solutions I could offer as well, such as you being in a Matrix type dream. But there is something more realistic that may help me prove my point. People have been known to have hallucinated people, things, and situations. A great example is in the movie A Beautiful Mind. A man spent most of his life thinking his college roommate was real, but in fact he had never existed in the first place. For all you know, this could be a mass hallucination, or like I said, dream. Because your brain will have created this whole environment for you, and there is nothing that is safe from being altered, because everything you ever know is the result of the brain processing it in the first place.