Plato described love as a serious mental disease. Aristotle saw it as a single soul inhabiting two bodies. Tina Turner dismissed the feeling as a second-hand emotion. The nature of love—how and when and why and with whom humans fall for each other—has preoccupied thinkers through the ages. Now a philosopher and a scientist have a go in two new and markedly different books.

In his latest work, “In Praise of Love”, Alain Badiou, a French philosopher, identifies three prevailing philosophical views of love. It can be an ecstatic encounter; an unsentimental contract; or an illusion, best treated with scepticism. He rejects all three. For Mr Badiou, love is the decision to live life through two perspectives, that of both the lover and the beloved. As such, it is more than the sum of its parts. Love “is a construction,” he writes, “a life that is being made, no longer from the perspective of One but from the perspective of Two.”

Mr Badiou sees risk as central to love. A loving relationship demands multiple and shared perspectives, which always give rise to incongruences and tensions. He reserves special scorn for dabblers in internet dating, who evidently believe that the search for “a photo, details of his or her tastes, date of birth, horoscope sign, etc” will ultimately net “a risk-free option”. This is to neglect the very essence of love, according to Mr Badiou, which involves the presence of risk, the possibility of failure and the need for vulnerability.

The book's chatty style (it is based on a conversation with Nicolas Truong, a French journalist) lends a deceptive simplicity to the ideas within. Get to work unpicking these concepts and it soon becomes plain that, like many French philosophers, Mr Badiou sacrifices clarity for linguistic zip and sparkle. Nonetheless, he leaves the reader with an incisive overview of philosophical thinking on love, from Plato to Kierkegaard to Lacan.

Robin Dunbar's book, “The Science of Love and Betrayal”, is—perhaps surprisingly—easier to get to grips with. Dr Dunbar, a professor of evolutionary anthropology with a study in this week's science section (see article), is best known for “Dunbar's number”, the limit to the number of people with whom one can maintain stable social relationships. He laments that scientists have largely ignored the concept of love. In this book he bridges the gap between the biological explanations for humans' romantic behaviour and the psychological, historical, social and evolu- tionary contexts that help to shape it.

In particular, he is interested in why humans have developed such an affinity for “pairbonding”, despite the fact that strictly monogamous mating and rearing systems are not terribly advantageous in evolutionary terms. Monogamy is not unique to humans. What is unique, however, is the intensity with which the species falls in love. Nearly every human culture in history exhibits this complex sense of longing, Dr Dunbar observes.

To understand this predisposition for monogamy, he takes readers through the myriad feelings of love, from the heady, breathless exhilaration of falling, to the stubborn persistence of familial affection, to the bitterness of betrayal. Throughout the book Dr Dunbar excels at taking obvious and familiar information—men prefer curvy women; women prefer men who dance well; older women rarely reveal their ages in lonely-hearts columns—and explaining the complex and often unexpected evolutionary science that lies behind it all.

Love is a journey, a game, a many-splendoured thing. Though some give it a bad name (if Jon Bon Jovi is to be believed), the rest of us find the subject endlessly fascinating. The struggle to understand such a mystifying phenomenon invariably requires the help of philosophers and scientists, and others besides. Good news for Mr Badiou and Dr Dunbar.