What makes a good wolf? For starters, sharp teeth. Getting meat off carcasses is hard without them. A warm coat of fur would go nicely too. It’s hard to hunt when you’re freezing. And of course a piercing howl. You’d need to let the rest of the pack know about that crippled deer limping along the brook.

Could this help answer the question “What makes a good human being?” Surely not. These are wolves after all, animals who spend much of their time licking themselves. We don’t do that. And we don’t have sharp teeth either. Warm coats? Not necessary. If it gets too cold, we can always borrow a wolf’s.

The moral philosopher Richard Kraut doesn’t, thankfully, deny any of this in his “What is Good and Why: The Ethics of Well-Being” (Harvard University Press, 273 pages, 435). But he does think that reflection about how to live a human life should start from the fact that we can use the word “good” to refer to any number of things (wolves, knives, etc.), and that when we do so we are saying roughly the same thing each time. The things that are good for a wolf are the things that help it flourish in its particular wolfish way. The things that are good for a knife are the things that help it flourish in its knife-ish way. The things that are good for human beings, then, are the things that help them flourish in their particular human way. Poetry may well be a good thing, but not for a wolf, given the kind of thing it is. The wolf manages just fine without it.

This, in very broad strokes, is Aristotle’s argument in the “Nicomachean Ethics,” and Mr. Kraut acts as a kind of amanuensis for Aristotle in this book. The central idea, both of Aristotle’s work and Mr. Kraut’s, is that we cannot know what is good for us without knowing, first, what “we” are. We need some account of the facts of human life, one that answers the questions “What are our distinctively human capacities?” and “How are they exercised?”

This, of course, is where things get tricky. It is one thing to specify what a flourishing wolf’s life looks like — lots of meat, a cozy lair, plenty of offspring — and quite another to specify what a flourishing human life looks like. “Using the categories of common sense,” says Mr. Kraut, “we can say at least this much: A flourishing human being is one who possesses, develops, and enjoys the exercise of cognitive, affective, sensory, and social powers. Those, in broadest outline and roughly speaking, are the components of well-being.” This means, Mr. Kraut continues, “that there is one kind of life that is best for all human beings — a life of flourishing, one that follows a pattern of psychological and physical growth, filled with enjoyment.” The answer to the interrogatives of his book’s title comes in two parts: What is good is what helps us flourish; why it is good is because a flourishing life is the life “best for all human beings.”

This talk of a single life that is best for “all human beings” makes modern liberals who follow the philosopher John Rawls very nervous. If there were a single best life, why wouldn’t we coerce people to lead it? That only seems sensible — but also terrifying. Thankfully, says Rawls, there is any number of ways to lead a flourishing human life: as an accountant, an athlete, or an aesthete. To claim otherwise is to misunderstand certain very obvious facts about modernity. Rawls’s answer to the two interrogatives takes a much different shape. What is good is the “satisfaction of rational desire … [a human being’s good] is determined by what is for him the most rational long-term plan of life.” Why is it good? Quite simply, because we desire it.

But, continues the Rawlsian, to ask after what is “good” is to mistake what ought to guide human life — namely, the ‘ought’ itself, or what Rawls calls the “concept of right.” Our lives and institutions need to take their bearing not from the idea of good but from an idea of what ought and ought not to be done, always and everywhere. Rawls himself was a famously generous reader, and he would have found much to savor in Mr. Kraut’s book. But he could savor it only because he had already polished off the less tasty, but more nutritious main course: his magnum opus, “A Theory of Justice.”

Mr. Kraut’s book is a moral menagerie, its pages a teeming habitat filled with plants and animals no less than humans. It is only a minor stretch to say that a slightly dim library shelver could mistakenly file this book under “horticulture” or “zoology” and not “philosophy.” Some might take this as a sign that something has gone seriously wrong — that we are, we might say, howling up the wrong tree.

“This is a good orchid,” “This is a good wolf,” and “This is a good man” are, quite obviously, grammatically similar. Should we follow Mr. Kraut and conclude that the word “good” functions identically in each sentence? He does make a persuasive case for placing the idea of “flourishing” — and not the “right” or the “moral” — at the center of human life. We, like orchids, need the nurturing attention of others to grow. This is good for both species. We, like wolves, may even lick one another from time to time. This, too, is good for both species. And all these things are good independent of what we desire. But perhaps the place to start is not with similarities but with differences. We have reason. And we use reason determine what is right and bind ourselves to laws shaped (hopefully) by what is right. And no orchid or wolf has done that yet.

Mr. Boyle is a writer in Chicago.