From a New York Times article asking, "What do you believe is true even though you cannot prove it?"

David Buss
Psychologist, University of Texas; author, "The Evolution of Desire"

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.