Irving Singer's trilogy The Nature of Love has been called "majestic" (New York Times Book Review), "monumental" (Boston Globe), "one of the major works of philosophy in our century" (Nous), "wise and magisterial" (Times Literary Supplement), and a "masterpiece of critical thinking [that] is a timely, eloquent, and scrupulous account of what, after all, still makes the world go round" (Christian Science Monitor).

In the first volume, Singer begins by studying love as appraisal and bestowal as well as imagination and idealization. He then examines the contrasting views of Plato, Aristotle, Plotinus, Ovid, Lucretius, Saint Augustine, Saint Thomas Aquinas, and Martin Luther. After having described the nature of erotic idealization, Singer analyzes the religious idealization in Judeo-Christian concepts of eros, philia, nomos, and agape. Medieval Catholicism sought to combine these four ideas of love in the "caritas synthesis." Luther repudiated that attempt on the grounds that love exists only in God's agapastic bestowal of unlimited goodness upon humanity and all of nature. In relation to the different modes of theorizing, Singer explores the humanistic implications of each.