Alan Lightman: “One of my longtime friends is Rebecca Goldstein. Like me, she’s a scientist and a novelist. She started studying physics at university, but switched to philosophy. One of the things she wrote really speaks to me: ‘Science tells us what is. Philosophy tells us what matters.

Rebecca Goldstein: ‘It’s a serious business… being human is a serious business.’

We spoke on an outdoor deck, high up in the Berkeley Hills in California, where she was spending several months.”