Though the very notion of love is embedded in the word “philosophy,” we don’t usually turn to the philosophers for an understanding of that most elusive and baffling of emotions. Plato’s ascending scale of transcendent, suprasensual love, as expounded by Diotima in the Symposium, is certainly exalted, as is Spinoza’s amor Dei intellectualis, but neither is likely to come readily to mind when we find ourselves caught in the grip of passion or even of simple affection. Socrates ogles Alcibiades and his desire is convincing because timelessly human. But generally when philosophers, and academic philosophers in particular, discourse about love or passion, the result is faintly comical. When the cerebral meets up with the instinctual, we are more apt to recall poor Emil Jannings in “The Blue Angel” and his relentless mortification at the hands – or should I say, the high-heels? – of Marlene Dietrich than to be philosophically enlightened.

That such a reaction may be downright unfair as well as wrongheaded is elegantly demonstrated in the American philosopher Harry G. Frankfurt’s new book “The Reasons of Love” (Princeton University Press, 101 pages, $19.95). Mr. Frankfurt is an emeritus professor of philosophy at Princeton, but you wouldn’t guess it, for he writes clearly and beautifully. His little book provides the rare pleasure of witnessing an agile and sensitive mind grappling with an issue of universal importance.

Mr. Frankfurt’s work is divided into three essays, based on a series of lectures he gave at Princeton, so the argument has a syllogistic structure. In the first essay, he ponders the fundamental question “How should we live?” Living in accord with morality, while good in itself, cannot provide an answer, he argues. Rather we must ascertain what is truly important to us; that is, what we actually care about. Caring about something may confer importance on it. Since I care about the Red Sox, their recent triumph is important to me (forgive me, Yankees fans!). But were I uninterested in baseball, their winning or losing would be a matter of indifference. This disposition to care about something lies at the heart of Mr. Frankfurt’s argument; as he puts it, “caring about something is essential to our being creatures of the kind that human beings are.”

In his second essay, he considers love itself. He is less interested in romantic love which involves, as he wryly says, “a number of vividly distracting elements.” In parental love, by contrast, he finds his most apposite model, for this “is the species of caring that comes closest to offering recognizably pure instances of love.” The salient characteristic of parental love is disinterestedness. By this he means that parents don’t base their love for their offspring on a rational calculation of some benefit their progeny may bring them; they love them for themselves, and they want what is good for them.

In one of his finest passages, Mr. Frankfurt argues that we don’t love our children because of their assessed worth as individuals; on the contrary, it is our love that confers special worth on them, quite apart from whatever merits they may possess. As he explains, “It is not because I have noticed their value, then, that I love my children as I do. Of course, I do perceive them to have value; as far as I am concerned, indeed, their value is beyond measure. That, however, is not the basis of my love. It is really the other way around. The reason they are so precious to me is simply that I love them so much. “This isn’t the tautology it seems. The fact, and the act, of loving one’s children are as crucial to parents as the children are in themselves. The love that Mr. Frankfurt describes is far from the usual philosophical abstraction; rather, such love is involuntary, exceedingly particular, and not susceptible to substitution. We don’t choose to love our children; we love them more than the children of others; and we cannot replace them.

Mr. Frankfurt’s final chapter offers a persuasive defense of self-love or love of what he calls, borrowing a term from Kant, “the dear self.” He shows – irrefutably, I think – that self-love is at the root of any meaningful existence. This is no argument for selfishness but the recognition of a primal truth about human nature; why, he asks, would we be commanded to “love our neighbor as ourselves,” if self-love were not natural? For the final “reason of love” is that love, and love alone, confers significance on life, and that conferral of worth begins in the individual self. “Rationality and the capacity to love are the most powerfully emblematic and most highly prized features of human nature,” he notes. By a striking paradox, rationality and love, which constrain us severely – the one to logical conclusions, the other to an involuntary and particular commitment – allow us the greatest measure of freedom. Despite, or because of, their constraints, both reason and love afford us “an experience of liberation and enhancement” as well as “the invigorating release and expansion of ourselves.”

The notion of self-love as the type of all authentic love is not new. We find similar conceptions in such ancient texts as the Upanishads as well as in Medieval Islamic mysticism; the great Persian Sufi al-Ghazali, who died in 1111, argued that self-love and self wonderment are essential for a love of God since “man is the most amazing of creatures and yet he remains unamazed at himself” and “he who knows himself knows his Lord.” That does not lessen Mr. Frankfurt’s originality in this rich little book. For him, not what we love but the fact that we love defines us as human beings. “The function of love,” he writes, “is not to make people good. Its function is just to make their lives meaningful, and thus to help make their lives in that way good for them to live.”

Mr. Frankfurt also considers the inability to love, the problems of boredom and meaninglessness in the world, and even the impossibility for many of finding their inmost selves lovable (he recommends a good sense of humor as an antidote). “Love thyself!” may not be as obvious as the ancient Delphic Oracle’s “Know thyself!” but Mr. Frankfurt has persuaded me that it is equally essential.