The reality-based community is comprised entirely of people who think too much with their head and not enough with their gut.
The term comes from an Unnamed Senior Adviser to George W. Bush as quoted by journalist Ron Suskind.
He said that guys like me were "in what we call the reality-based community," which he defined as people who "believe that solutions emerge from your judicious study of discernible reality." I nodded and murmured something about enlightenment principles and empiricism. He cut me off. "That's not the way the world really works anymore," he continued. "We're an empire now, and when we act, we create our own reality. And while you're studying that reality -- judiciously, as you will -- we'll act again, creating other new realities, which you can study too, and that's how things will sort out. We're history's actors . . . and you, all of you, will be left to just study what we do." 
That Unnamed Senior Adviser? Stephen Colbert.