Armed with little more than the principles behind them and some maths that has been solved for half a century, you can build an entire theory of how to invest your hard-earned savings. It tells you how much you should risk on the stockmarket, how much you should keep safe and what you can spend in retirement. It requires just a few facts about financial markets and some of the simplest, cheapest investment funds that asset managers offer. What is more, it was first formulated by a Nobel-prizewinning economist and is today almost universally accepted by his successors as the “correct” approach to investment and spending. Yet outside academia, almost no one has heard of it.