By no means am I a mathematical giant or even a person of intense scientific interest or curiosity. What I do believe in is only three dimensions. Some say time is a dimension but I believe it is just part of the dimension of length. As far as flat land is considered, I find it harder to believe in a 2D world than a 4D world. No matter how flat or small, all the way to the atomic level there are still three dimensions. I saw the flat land scenario on youtube and how if a 3D being were to touch down the flat land people really wouldn't realize it but the 3D being would still be there. But using flat land as an example is, at least in my mind, not good idea. What I do believe is that the 3 dimensions may have unlocked properties that we don't know about. I believe everything must happen within the three dimensions. For example, what makes time travel going into any other dimension other than the 3D one we live in. Time travel may just be a special property of the lenghth dimension. I also can not fathom a 4th dimension. Maybe you can, but I'd like to see anyone explain so it makes sense. Maybe the supposed properties of the "4th dimension" that we can't see or experience are here in the 3rd dimension, but as a species we are just not sophisticated enough to tap into the properties of what lies within our three dimensions. What about dark matter? Does it exist in a 3D world? Is gravity part of the 3D world or does it extend into the 4D world? Does gravity have an effect on 4D beings if they exist? Do I totally doubt the existance of the 4D world? No. I can't just deny the existence of it, because maybe it does exist. Like I said before, so far, I believe there are only 3 dimensions.