No, I don't mean physical reality. Obviously, there is a real world "out there" that we all share. However, you may not realize the degree to which much of what you value is some other person's vision of what matters. This is an important thing to realize, if you are young (say, less than 40 years of age); because, one day the torch will be passed to you -- it will then be your burden to help others find meaning, and that usually entails projecting your meaning into the world for them to build on.
This all sounds very abstract, so let me make it a little more concrete: when I was younger, I delighted in trying to solve problems by esteemed mathematicians from an earlier era. Many of them left problems for later generations to solve, the solutions of which required pushing the boundaries a little further. The challenge in producing good problems like this is to find ones that are neither too difficult nor too easy, that are right at the boundary between the two.
As I became older, I came to realize that what looked to be problems of unassailable interest, were actually merely products of the culture and age in which they were written. An alien life-form might not find them at all interesting; in fact, might have a different cognitive style altogether, and find them "trivial", or merely "random" ("why would anyone find that random problem so interesting?"). Thus, math is more firmly a subject of the humanities than is often realized. There is objective truth in applying the rules of logic; but deciding what to apply it to -- deciding what matters -- is not so clear-cut.
And now I'm often asked to come up with interesting problems, and I'm always at a loss for what to say when people look up to me. It's not a burden I wish to bear; and yet, the alternative -- turning my back on it -- leaves the world feeling a little emptier.
Perhaps another way to describe it is that we're all playing Santa Claus in some way or another, wrapping the world in mystery, hiding the cold, dark, empty void that people aren't ready to deal with. Even people who say, "Let them invent their own meaning," don't really mean it -- because they invariably invest in a community and sense of meaning and purpose already designed for them, whether it be "progressive politics", "mathematics", or what have you. Truly inventing ones own, unique meaning is often isolating, and the greater the distance from what came before, the more isolation that is required.
When you get to be my age (not far from 50 years old), you begin to see the void more and more. Alan Watts spoke of this often, incidentally, but using different language. You begin to see how much value people place in ideas and problems and solutions that were dreamed-up by people you knew -- or knew of. I have the urge to reveal there is no Santa Claus, and say, "Yeah, he dreamed that problem up in a bar in Toronto back in the year 2000, drunk. You realize it's arbitrary, right? What, you think if you solve that problem that God will show up or something?"
But maybe none of this means anything to any of you, because you're not old enough yet. You haven't yet found out there is no Santa Claus; or, if you have, you don't really feel it yet in your bones the way that I do.
Ahhh.... how I wish I were 20 years old again, or even 30, and had my memory wiped, so I could feel the world had meaning again. I remember all the nihilistic messages when I was that age, and nodded at hearing them; but it never really penetrated the way that it does now. I suppose I will have to bear the torch passed to me a while longer, and share my meaning while I still exist. It won't cover up the emptiness, but is the closest thing to it within my power.