For the 20 centuries between Plato and Kant, the study of beauty was a signature concern of philosophy. Recently, however, beauty has largely been dropped from the philosophical curriculum. The story of how this has come to pass is inseparable from a broader story about how philosophy itself has become so hopelessly professionalized. Contemporary philosophers, preoccupied with their small quarrels, have abandoned the discussion of beauty to the likes of Elaine Scarry and Denis Donoghue and their colleagues in art and English departments. It should come as little surprise, then, that beauty has been smuggled back into philosophy by Alexander Nehamas, a professor of philosophy at Princeton, whose previous books have made wideranging inquiries into what he calls "the art of living." Mr. Nehamas is among his generation's few heirs to a philosophical tradition in America  which runs from Emerson and William James through Richard Rorty and Stanley Cavell  that believes it might have something useful to say to an educated public.

For Plato, beauty was inseparable from eros, or desire. Roughly with Kant, however, philosophers became increasingly "mistrustful of passion" for all the usual reasons: its baseness, unreliability, and emotional illogic. Kantian philosophy sought to eliminate passion from beauty, replacing it with a model of disinterested contemplation. Kant called this category "the aesthetic," which Mr. Nehamas interprets as "pleasure bereft of desire." Mr. Nehamas's brief but reverberant new book, "Only a Promise of Happiness" (Princeton, 186 pages, $29.95), proposes that we once again talk about beauty as "identical with the spark of desire." A beautiful object is, quite simply, one that beckons us to it. In the phrase Mr. Nehamas has borrowed from Stendhal, beauty is a "promise of happiness," a hint that a closer relationship with the beautiful object might enrich our lives. When we are captivated by beauty  in a person or an artwork  we find ourselves following wherever that beauty may lead, well aware that we might be forever changed by the encounter.

Mr. Nehamas's prose is so slyly witty ("I can remember, and I hope you do too, how lovely it felt when I began to hate Hermann Hesse") that it's easy to underestimate the implications of his argument. Not only professional philosophy but large swaths of culture begin to look different once we've included desire and uncertainty in our idea of beauty. When, for example, we talk about beauty in purely formal terms  as Modernist critics did we must conclude that beauty will always be a rare thing, its appreciation inherently difficult. But if instead we agree with Mr. Nehamas that beauty is identical to desire, that desire longs for engagement, and that such engagements are invariably risky, we might talk about beauty as we would talk about friendship: not as a verdict of something's worth but as indication that a relationship with the beautiful object will continue to give us unexpected pleasures over time.

Mr. Nehamas uses this deceptively simple idea  that any perspective on beauty requires what Proust called "this great dimension of Time"  to reframe the history of Modernism, to reconceive the role of the reviewer, and to demolish casually an array of philosophical and cultural distinctions: reality/appearance, physical/ mental, description/interpretation. To do justice to Mr. Nehamas's vision would require a book many times the length of his own. One example must suffice here: the putative distinction between high and low culture.

Mr. Nehamas devotes his longest chapter to Manet's "Olympia" and the effect the painting has had on him, but he also frequently invokes the television shows "Oz," "Frasier," and "St. Elsewhere." "Watching television and spending time thinking and even writing about it may well be ruining my character and wasting my life. Perhaps, though, you are missing something that could really add to yours," Mr. Nehamas writes. Because our most meaningful encounters with beauty unfold over time, we can only ever say in retrospect that a beautiful object has or has not made our lives  or our culture  better. "The perils of the popular arts, aesthetic and moral, seem greater because the jury, so to speak, is still out, and their place within culture is not yet determined."

The difference between high and low culture is thus spread out along a spectrum of how powerful  and how enduring  our experiences with beauty are. On one end is pornography, which provides ephemeral satisfaction of uncomplicated desires. On the other end is Proust, which Mr. Nehamas claims he literally could not imagine his life without. In the middle lie "Olympia" and "St. Elsewhere" and other preoccupations that hold our interest to varying degrees. We might one day find, as Swann did with Odette, that we have wasted decades in pursuit of an undeserving beauty. We might also find that years of sitting in front of "Frasier" reruns has proven more inspiring than we ever would have guessed.

Beauty is only ever that promise: There is no a priori judgment that might reveal what will prove evanescent and what sustaining. In seeking such a failsafe, the philosophical tradition since Kant has made aesthetics a science of indifference. By inventing "pleasure bereft of desire," it found itself unable to tell us anything meaningful about beauty or desire. In Mr. Nehamas's vision, the possibility of beauty is well worth the price of uncertainty.