The French philosopher Denis Diderot was once given a splendid scarlet dressing gown, and at first he was pleased. Before long, though, an odd unease set in. Next to the elegance of the new robe, everything else he owned — his shabby desk, his faded wall hanging, his plain chairs — suddenly looked unbearably worn. To bring the room back into harmony, he began replacing his old belongings one by one, and before he was done he had fallen into debt. The pattern, now called the Diderot effect, exposes something basic about how we buy.

We rarely acquire a thing on its own; we assemble a web of possessions that together signal who we are. Let one item break sharply from that established level, and the whole arrangement feels out of joint. The discomfort of the mismatch then presses us to raise everything else to meet it. A single fine acquisition, read this way, tends to       ?      .