Sex isn’t black or white (Image: Nils Jorgensen/Rex Features)

“I AM satisfied with a wild free Nature,” the psychologist-philosopher William James once wrote to a quarrelsome colleague. “You seem to me to cherish and pursue an Italian Garden, where all things are kept in separate compartments, and one must follow straight-ruled walks.”

I’ve always admired the way James challenged what he perceived as scientific dogma. In this case, he raised a conundrum we still wrestle with today. Science, with its love of classification, seeks to impose a strict order on the world around us. Yet life on Earth is (forgive the pun) by nature tangled, messy …