Thinking at the Margin
Water is essential and cheap; diamonds are frivolous and dear. The puzzle dissolves once you see that value is set not by categories but by the next unit — and the next glass of water is everywhere.
For a century the water–diamond paradox embarrassed economists, until marginal thinking resolved it: nobody ever chooses between all water and all diamonds. You choose one more glass or one more gem — and with water abundant, the next glass is worth almost nothing, while the next diamond is not.
Real decisions live at the margin. Not “is exercise good?” but “is one more session this week worth its hour?” Not “is staff valuable?” but “what will the next hire add?” Averages describe the past; margins decide the future.
Businesses price this way instinctively — an empty airline seat sells cheap because the cost of filling it is nearly zero. People who learn the same trick stop over-buying categories that were only precious in their first few units.
Decide on the next unit
Replace “is it worth it?” with “is one more worth it?”
For any recurring choice — hours, hires, features, helpings — ask what the next unit adds and what it costs. Continue while the addition beats the cost; stop the moment it doesn't.
Ignore the average, and the total
That the whole project was worth it says nothing about the next month of it. Evaluate the increment in front of you on its own terms, as if it were a fresh proposal.
Notice where your margins have flattened
First salary raises, first rooms, first wins transform a life; the tenth of each barely moves it. Redirect effort toward whatever is still steep for you.
Almost nothing in life is all-or-nothing. Everything is one-more-or-not.