We think science is based on facts and evidence. But from gravity to dark matter, string theory to parallel universes, its theories are curiously bereft of hard evidence. Is evidence less important than we think and conjecture alone capable of leading to greater understanding? Or has science dangerously drifted into fantasy?

A dialogue at the 2016 "How The Light Gets In" Festival, at Hay-on-Wye, with Rupert Sheldrake, philosopher Massimo Pigliucci and CERN physicist Tara Shears, chaired by Philip Ball. From iai, The Institute of Art and Ideas Philosophy for our times: cutting edge debates and talks from the world's leading thinkers.