John Brockman asked various scientists about what they believed in but couldn't prove; the NYT prints some of those answers, including a really sweet one by David Buss, a psychologist at the University of Texas (and here's the link for the article):
I've spent two decades of my professional life studying human mating. In that time, I've documented phenomena ranging from what men and women desire in a mate to the most diabolical forms of sexual treachery. I've discovered the astonishingly creative ways in which men and women deceive and manipulate each other. I've studied mate poachers, obsessed stalkers, sexual predators and spouse murderers. But throughout this exploration of the dark dimensions of human mating, I've remained unwavering in my belief in true love.
While love is common, true love is rare, and I believe that few people are fortunate enough to experience it. The roads of regular love are well traveled and their markers are well understood by many - the mesmerizing attraction, the ideational obsession, the sexual afterglow, profound self-sacrifice and the desire to combine DNA. But true love takes its own course through uncharted territory. It knows no fences, has no barriers or boundaries. It's difficult to define, eludes modern measurement and seems scientifically woolly. But I know true love exists. I just can't prove it.