You're a miner. You've been a miner all your life. You chip away at one meter square blocks and harvest the floating objects that appear. Sometimes you catch a glimpse of something--a void?--in the spaces between the blocks, but you can never learn anything about it, so you learn to accept it as just part of the fabric of your universe.
Using the materials you mine, you build houses and farms. You fight creepers and zombies. You sleep when you have to, and you eat to keep up your energy. You build cactus walls to protect your home. Your creations become more and more intricate as your complex sprawls. You visit the far lands and puzzle at their strangeness, though with what you know of your universe, the one you've inhabited as far back as you can remember, you feel deep inside that they are somehow mathematically, even philosophically inevitable.
Eventually, you build redstone computers on a massive scale.
Am I describing the life and times of Herobrine? Maybe. If we could imagine Herobrine as a brain-in-vat type intelligence, the example would fit quite well. What I'm trying to illustrate (and I'm aware that I'm not the first person to do this) is that Minecraft does, in fact, constitute a complete universe.
If we assume that our Herobrine has never received and can never receive any sensory input aside from appearing one day at his spawn point, and that as long as he continues to exist, the entire understanding of reality he establishes in his mind will concern only Minecraft, with no outside interference, then Minecraft is, indeed, a universe. We may say it isn't 'real', but who cares what we think. The experience we have of our own universe is precisely like this from our perspective. We only say it's 'real' because we don't know anything else. Our Herobrine has nothing to go on beyond Minecraft. To him, it's as real as this keyboard in front of me, and the universe as we experience it would be alien and inexplicable to him; overwhelming even--madness-inducing.
Where am I going with this? Bear with me. I'm trying to build a paradigm, and I'll continue on this topic in future posts. For now, it's St. Paddy's day and I have to go get soused. ;)