As we spend more, and as companies are pushed to invest, they say, "Hey wait a minute! There's more demand in the system. Let's invest more."

The world changes! So we're in a situation today where the only policymakers that have flexibility are central banks. But they don't have the instruments! So they've had to experiment, and the more you experiment, the more uncertainty and the higher the risk of collateral damage.

The theory is that if you take interest rates negative, people are going to say, "That's a silly game! I'm not going to lend my money to governments who want me to pay them. I am going to go into the stock market where I can get positive returns!"

We normally think if you're going to lend someone money, you should get some reward for doing this. In Europe, it's a tax!

If you happen to be the only one with negative interest rates, you also weaken your currency, which means you make your exports more competitive.

Once you start moving [market] lower, then you trigger of all sorts of things. You trigger people who have to sell because they're over-levered. So they sell their winners and their losers. They're just trying to raise cash. So, what you then get is spreading malaise throughout the global markets.

[When] the market is trying to get to terms with, first, lower global growth, particularly out of emerging markets and China. And, second, the market is worried the central banks have run out of ammunition. So put these two things together, and then investors are repricing the market lower.