Alfonso Sicilia Sobrino, a Spanish artist, recently gave us one of his prints, a thank-you gift for putting him up in our spare room for a couple of nights. (You can see some of Alfonso’s work by going to the Esfera del Arte website and clicking on his name in the ‘Our Artists’ section.) It was a very generous gesture, and one that we gratefully accepted. My wife and I both really liked the vivacity and cheerfulness of the piece, which we hung in the living room in a spot that used to be occupied by a clutch of black-and-white drawings. The print brightened up that whole corner of the room. But even as our recent acquisition cast the living room in an entirely new light, it occasioned other, somewhat darker thoughts.

The black outlines of dust on the wall where the old frames hung were now clearly visible, like the chalk lines around the body at a murder scene. We’d have to paint those, I thought. And that section of wall near the corner where the water damage was, we’d have to do something about that, too. It looked too much like that part of the room had some kind of strange skin disease. And that gash in the ceiling where the plaster fell down years ago; why the hell haven’t we fixed that yet? And I’m sick and tired of constantly stumbling over the lip of the stair where the carpet is worn away. It’s beyond carpet cleaning. Let’s get new carpets for the whole stairway while we’re at it. Yes, before my enthusiasm for the print had even cooled, I had succumbed to the dreaded Diderot effect.

The Diderot effect is named after the 18th–century French writer Denis Diderot, who spent 25 years editing the massive Encyclopédie, one of the founding documents of the Enlightenment. Diderot is also the author of a charming essay called Regrets on Parting with My Old Dressing Gown, in which he describes how the gift of a beautiful scarlet dressing gown plunges him into debt and turns his life upside down. Initially pleased with the unexpected gift, Diderot describes how he soon came to rue his new garment. Compared to his elegant dressing gown, the rest of his possessions began to seem tawdry. His old straw chair, for example, just wouldn’t do. So he replaced it with an armchair covered in Moroccan leather. And the rickety old desk that groaned under his papers; that was out, too, and in came an expensive new writing table. Even the beloved prints that hung on his walls had to make way for newer, more costly prints. “I was absolute master of my old dressing gown,” Diderot writes, “but I have become a slave to my new one … Beware of the contamination of sudden wealth. The poor man may take his ease without thinking of appearances, but the rich man is always under a strain.”

Consumer researchers call this kind of trading up “the Diderot effect.” But Diderot was also a marvellous aphorist and it is he who is responsible for coining that classic French phrase l’esprit de l’escalier: ‘the spirit of the staircase,’ that moment of belated inspiration when you think of the perfect comeback for a difficult encounter only when you’re walking down the stairs after the conversation is over. That’s another Diderot effect I observe too often in myself.

Diderot, though, didn’t suffer much from l’esprit de l’escalier. He was famed as a brilliant conversationalist, and seems to have devised his bon mots while coming up the stairs rather than going down them. During the 25 years or so he spent editing the 28 volumes of his Encyclopédie, he also wrote hundreds of entries on a bewildering array of topics in agriculture, industry and science. His aphorisms all promoted freedom of thought, religious tolerance and the importance of scientific inquiry:

From fanaticism to barbarism is only one step.

The first step towards philosophy is incredulity.

In order to shake a hypothesis, it is sometimes not necessary to do anything more than push it as far as it will go.

Diderot is a classic Enlightenment figure: the optimistic skeptic. He doubted pretty much all the received wisdom of his own time but, like Mr. Micawber in David Copperfield, he was sure that something better would turn up thanks to human inventiveness and ingenuity. Mr. Micawber was also seemingly immune to l’esprit de l’escalier and like Diderot had some insightful things to say about economics. Mr. Micawber’s equation for financial happiness, for example, really can’t be rivalled:

Annual income twenty pounds, annual expenditure nineteen nineteen six, result happiness. Annual income twenty pounds, annual expenditure twenty pounds ought and six, result misery.

It’s frustrating, if not exactly misery-inducing, not to be able to afford the home improvements our new print seems to deserve. And I’ve been trying to come up with reasons why the Diderot effect should not apply to me, but so far without success. I’m sure I’ll think of something while I’m walking down the stairs…