[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.

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!"

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

Investors should invest on what they know. The biggest mistake is to invest on what they don't know.

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

It is hard to imagine that, having downgraded the US, S & P will not follow suit on at least one of the other members of the dwindling club of sovereign AAAs. If this were to materialise and involve a country like France, for example, it could complicate the already fragile efforts by Europe to rescue countries in its periphery.

How long can interest rates stay negative? Think about this. Not only are you lending your money to governments, but you're paying them interest for the privilege of doing so.