“Judge the value of what you have by what you had to give up to get it.”

The principle of an opportunity cost does not at first glance seem hard to understand. If you spend half an hour noodling around on Twitter, when you would otherwise have been reading a book, the lost book-reading time is the opportunity cost of the tweeting. If you decide to buy a fancy belt for £100 instead of a cheaper one for £20, the opportunity cost is the £80 shirt you could otherwise have bought. Everything has a cost: whatever you were going to do instead, but couldn’t.

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We would make better decisions if we reminded ourselves about opportunity costs more often and more explicitly. Nowhere is this more true than in the case of time. Many of us have to deal with frequent claims on our time — “Can we meet for coffee so that I can pick your brains?” — and find it hard to say no. Explicitly considering the opportunity cost can help: if I meet for coffee I’ll have to work an hour later, and that means I won’t be able to read my son a story before bedtime.