One thing I feel I should be clear about up front: I have always liked “Beavis and Butt-Head.” You might not expect that a fondness for the 1990s MTV cartoon series, with its grunting, cretinous heroes, would be germane, one way or the other, to appreciating the arguments offered by the moral philosopher, Susan Neiman, in her high-minded “Moral Clarity” (Harcourt, 467 pages, $27). But look in the index to the book and you will find Beavis and Butt-Head, right after Pierre Bayle, the great 17th-century atheist — a kind of company they have surely never enjoyed before.

Beavis and Butt-Head loom surprisingly large in Ms. Neiman’s worldview, as the living (or at least animated) fulfillment of Nietzsche’s prophecy about “the last man,” the morally depleted figure he feared would populate a world deprived of God and moral struggle. “One is clever and knows everything, so there’s no end of mockery,” Nietzsche predicted; to which Beavis and Butt-Head offer an enthusiastic “Heh heh.” The characters, to Ms. Neiman — a professor of philosophy, the director of the Einstein Forum in Germany, and the author of “Evil in Modern Thought” — stand as an “emblem of nihilism.” “It’s hard to imagine Beavis and Butt-Head kicking or screaming about anything,” she writes. “Everything related to values — desires and goals and aims and dreams — has disappeared from their world…. Their world is never graced by a shadow of reverence.”

Well, yes, that is the point of comedy: to mock official pieties, to offer a liberating space where reverence is suspended. A world made in the image of comedy would, it is true, be intolerable to live in. How long could we stand it in the forest of “A Midsummer Night’s Dream,” where men grow donkey ears and lovers constantly forget who they are supposed to love? How could we breathe wedged in a stateroom with the Marx Brothers, having everything we say turned to nonsense by Groucho’s puns? But equally, how could we stand to live in a world that we couldn’t occasionally escape or thumb our noses at? Beavis and Butt-Head are not an ideal, as Ms. Neiman surely knows, but a joke; and an ideal so vulnerable that it cannot take a joke is not vigorous enough to guide us.

This is the central dilemma — not just an intellectual but an aesthetic one — that “Moral Clarity” never quite manages to resolve. Ms. Neiman’s purpose, she declares, is to give the ailing left, especially its collegiate division, the confidence it needs to reclaim a moral vocabulary she believes has been usurped by the right. “Most of the voices willing to speak in universal moral terms at all,” she writes in her introduction, “now consider themselves conservative…. However shabbily [the right’s] partisans may behave in private, they offer a public conception of goodness the left no longer knows how to defend.”

Most of the left would certainly refuse to recognize itself in this portrait. From “Justice for Janitors” to “Not In My Name,” the most vigorous left-wing organizations have no problem claiming the language and “conception” of morality, whether or not their claims deserve to be granted. What Ms. Neiman is really talking about, one suspects, is the academic left, among which she has spent her professional life. Here, in the precincts where essentialism is a dirty word, the hermeneutics of suspicion are second nature, and Foucault is the sacred doctor, it is indeed hard to use the old, large words without air quotes: words such as heroism, goodness, and Enlightenment.

These are the benighted souls to whom Ms. Neiman carries moral clarity; and because she combines the didacticism of a professor with the righteousness of a missionary, she can hardly avoid the chilling tones of condescension. When, using one of her favorite devices, she italicizes a hypothetical protest to her argument — “If being a hero is a matter of small steps and everyday distinctions, it’s hardly what it’s cracked up to be” — her rejoinder is disastrously parental: “You know what? Neither is being a grown-up.” It is just possible that a college student would accept this without bridling. Actual grown-ups, who are accustomed to being spoken to as equals, will surely find it intolerable.

Yet while she has not found the right tone for inculcating her message, Ms. Neiman is right that the duty of maturity — of living with ambiguity and complexity, of sustaining ideals despite disappointments — is, in fact, a crucial part of enlightened morality. Liberalism has earned a reputation as a shallow, hypocritical, bourgeois creed, as Ms. Neiman complains: “If you’re looking for something to unite contemporary thinkers across every possible spectrum, you can do no better than to invoke the specter of the Enlightenment monster: a beast filled with icy contempt for the instincts and driven by blind, dumb optimism or protototalitarian lust for domination. The monster is relentlessly cheerful, stupendously gullible, and endlessly naïve.”

But as Ms. Neiman argues, neither liberalism nor the Enlightenment that formulated it are so contemptible, or so dispensable. At its best, liberalism is a genuinely tragic understanding of human nature, because it combines a recognition of our capacity for evil with a recognition that we ourselves are the only possible source of good. Liberalism is the art of limits, and the virtues it teaches are ironic ones: restraint, toleration, patience, civility. These are so unlike the classical virtues of bravery and piety that it takes a long education for a person, or a civilization, to recognize that they too can be glorious.

By far the best part of “Moral Clarity” is its middle section, where Ms. Neiman, drawing on her expertise as a historian of philosophy, reminds us of the way the West received that education in liberal virtues, from the thinkers of the 18th-century Enlightenment. Her portraits of Hume, Rousseau, Voltaire, and above all Kant, are reminders that these philosophers — so easy to malign from either the right or the left — deserve to be cherished as what she calls “enlightenment heroes.”

She argues, for instance, that Voltaire, whose hatred of organized religion is well known, was nevertheless a sincere votary of “natural religion,” capable of awe before Creation and a ferocious dedication to morality. His disdain for Catholicism was not a disdain for God, but for the human perversion of the divine: “Voltaire called the Christian image of God blasphemous; Kant thought it idolatrous. A God who could be bribed with behavior that was good for the sake of reward was no better than a god who could be bribed with the smoke of a sacrifice…. their God looks more like a Mafia boss than a God of justice.” Kant’s categorical imperative, whose moral sublimity entrances Ms. Neiman, was a way of understanding right and wrong that made the biblical promise of reward and punishment seem positively insulting. It is Kant, in fact, who gives Ms. Neiman her metaphor of morality as a process of growing up.

The problem with “Moral Clarity,” which goes far to vitiate the inspiration of its historical insights, is that Ms. Neiman cannot translate this Kantian sublimity into her own language and situation. She is right that human beings need a demanding moral code and a vision of transcendence if they are to find the pursuit of nobility and goodness possible. But she fails completely to show that her own politics — a conventional, Bush-obsessed leftism — are that transcendent morality. They remain, instead, a habit — “left,” she writes, is “where when all’s said and done the heart beats” — and there is nothing in “Moral Clarity” that could change the political beliefs of anyone who does not start out sharing Ms. Neiman’s premises. What she offers instead is a series of earnest clichés: “We are as good as we need to be in order to act as if we are”; “If you believe that progress is possible, you can do something about global warming.” As intellectual therapy for the disheartened left, this may be useful. But therapy, as “Moral Clarity” amply proves, is not philosophy.

akirsch@nysun.com