What is reality? Why, I'm surprised you would even ask. Factol Darius from the Sign of One is more than happy to supply the answer:
"The planes exist because the mind imagines them. We are all merely constructs of another's thought, existing purely in a world created by someone's mind. I know that I have created this Multiverse, you had better hope that I do not forget you. Welcome to my Mutliverse. You're everything I'd imagined you'd be."