So, the sky is falling in, blood on the streets, brokers leaping from buildings in a single bound.... The world is shouting 'sell'.
I think to myself two things: 'What's fundamentally changed in these businesses compared to last week?' and 'buy'.
I don't mind (other than psychologically) if the price drops further - I'm buying for the next 40 years..... though it's nice to buy at the bottom, the only way to reliably do this is to be lucky.
If I'd have bought something last week, I'm even happier to buy it now. If I'm holding something, I'm happy to buy more (if I can!)... what's better? Two cans of baked beans for 20p, or two cans for 10p? Everything else being equal, the latter. So it's a bit amazing to see joe public happy to buy shares when they're expensive, but scared when they're cheap.
Essentially, the share price is (short term) governed by emotion, long term by results. The trick is to buy good business from panicky investors, and then wait, or alternatively be really, really, good at spotting the way they're about to panic and buying the emotion - the latter is virtually impossible to do reliably, in my opinion.
The long and short of it is that I quite like periodic market tumbles, as long as they happen when I've got some cash!
Unfortunately, that isn't really the case right now.