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!"

America's downgrade may serve as a wakeup call for its policymakers. It is an unambiguous and loud signal of the country's eroding economic strength and global standing. It renders urgent the need to regain the initiative through better economic policymaking and more coherent governance.

As a whole, investors should welcome attempts to safeguard the integrity of markets. You need very clear rules applied to markets.

The once-unthinkable loss of the AAA rating will constitute a further hit to already fragile business and consumer confidence.

[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.

Investors have to ask themselves two questions. How much can we grow our investments? And, can we afford our mistakes?