What a propitious time to be coming out with your book, Dan. After all, the entire world is in a bubble! That is, according to veteran investor Jeremy Grantham, who recently wrote a letter to his investors titled: "It's Everywhere, In Everything: The First Truly Global Bubble." It's like the Matrix, man.

In your book, you finish with discussions of two potential contemporary arenas of bubblicious activity: housing in the U.S. and the alternative energy sector. But I'm with Grantham. I don't think you went far enough.

Those are only two manifestations of the global bubble, which stretches from art to commodities to housing and beyond, into all sorts of financial assets. We are seeing record multiples on private equity deals, as leveraged buyout firms, flush with cash and easy access to borrowing, have bid up prices of all sorts of companies to take them private. Prices of junk bonds are at records and stock markets around the world range from deceptively expensive (the U.S.) to outlandish (The Shanghai market).

New, innovative, little-understood and unregulated derivative products have allowed lightly regulated hedge funds to make highly leveraged investments across all markets. Central bankers and financial regulators openly worry that they don't understand these developments and cannot measure how much leverage is out there. Welcome to "Planet Leverage," as my former colleagues at the Journal, Randy Smith and Susan Pulliam, called it in a recent Page One piece.

At first when I read that Grantham piece and the WSJ article, I was worried. As you have pointed out, I gravitate toward the doom-and-gloom. So I was glad that you and your book set me straight. The global bubble must be a good thing! Such a relief.

Unfortunately, I'm just a journalist and I keep falling back into bad habits of mind. I keep worrying that bubbles need to pop first. I fret that it is only after all those silly investors have poured all that money down the drain can we start to see what wonderful things the wreckage has bestowed upon us.

Judging from your first posting, I see you seem to be troubled too, at least about the real estate bubble. You ask: "What's the physical and mental infrastructure that gets left behind" from the real estate bubble?

Well, that was going to be my question. I just don't see it.

What, exactly, will the infrastructure or economic benefit from a housing bubble be? When Americans borrow excessively to buy bigger houses that they cannot afford, how does that benefit the economy? Where do the productivity gains come from? I think that's the main concern I have with your book.

You make an attempt at an answer in the book. You write about a handful of people moving from California to Kansas and some pricing efficiencies from the Internets. When the Florida condo market crashes, college kids are going to get cheap dorm rooms.

This seems like thin gruel.