"You don’t know what sort of rollercoaster rides the economy will take your retirement savings on, and you also don’t know what rate of inflation will persist through your lifetime. Will a box of eggs cost $6.00 a dozen when you’re 65, or will it be closer to $60? How can we possibly know how much money we will need to live on in retirement?... Inflation eats 3% on average, leaving you with 4% to spend reliably, forever."
And then MMM goes on to mathematically justify that 4% number: http://www.mrmoneymustache.com/2012/05/29/how-much-do-i-need...