If I had to choose a favorite scene in Tom Stoppard's The Coast of Utopia, which LCT produced to Tony-bestrewn grandeur in 2006-2007, it would probably be the one at the end of the first act in the trilogy's second part, Shipwreck. The writer Alexander Herzen and his family are living in Paris. Friends come to call. Among them is the literary critic Vissarion Belinsky, who, suffering from consumption, has little time to live. As a final, beat-back-death splurge, he has bought himself a sumptuous dressing gown. As Shipwreck's first act ends, he leaves for the train station, to head back to Russia, but forgets the dressing gown on a chair. Mrs. Herzen grabs the parcel and runs after Belinsky and company. The pleasures of life, as well as its dreams, are being left behind.

My favorite scene in When the Rain Stops Falling, which is as heart-catching as its counterpart in Shipwreck, also involves a down-at-heels character who acquires a dressing gown. I am not going to spoil the effect by naming the person. I will say, however, that the garment's appearance is presaged in the play in a speech of Elizabeth Law (the younger version), played by Kate Blumberg.

Elizabeth is talking about what can happen when any one of us acquires a bright, shiny item - an article of clothing, a piece of furniture - and takes it home. Its luster can suddenly put everything else in our house or apartment to shame. As a highly intelligent, progressive person, Elizabeth knows just which writer expressed this phenomenon in prose: Denis Diderot.

In "Regrets on Parting With My Old Dressing Gown," written around 1772 but published posthumously, Diderot describes how the gift of a beautiful scarlet dressing gown plunges him into debt and despair and turns his world upside down. Initially pleased with the unexpected gift, Diderot comes to rue the purchase. His old straw chair, for example, will no longer do. So he replaces it with an armchair covered in Moroccan leather. An old desk is discarded for a pristine new writing table. "I was absolute master of my old dressing gown," Diderot sighs, "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."

I always grow suspicious when a philosopher, who by virtue of intellectual gifts alone is blessed by a rare wealth, cocks an eyebrow at material good fortune. Such attitudinizing makes make me think of a line from Richard Greenberg's astute, underrated play Hurrah At Last. In response to some twaddle about how money can't buy one happiness, one character replies, more or less, "Money can't buy you happiness, but it upgrades despair so beautifully."

But back to the dressing gown in Rain. I'm taken with how the production's costume designer, Clint Ramos, came up with a sartorial equivalent for Diderot's remark. I asked Ramos, who is also represented in New York right now by his superb set-and-costume work in Jon Marans' striking off-Broadway play, The Temperamentals, to describe his approach to Rain's raiment.

Ramos replied that the dressing gown is in cashmere and was built for the show. "We originally had a darker red one that we got off-the-rack but we decided to go brighter."

And what were Ramos's clues for the garment? "It's all in the text," he replied. "Andrew Bovell [echoing Diderot] was very specific about the color - SCARLET. It's a great device to just cut through all the layers of drabness that accumulate as the play progresses. I wanted the gown to just be a classic shape, with no overt signs of futurism but something that could conceivably be found 30 years from now. Because it's scarlet it already calls attention to itself: there was no need to punctuate the obvious."

The play's dressing gown may not punctuate the obvious, but it pops out of the play in a surpassingly beautiful, emotionally and intellectually resonant way.

BRENDAN LEMON is the American theater critic for the Financial Times and the editor of lemonwade.com.