Do we have moral duties to people who don’t yet exist? By grappling with puzzles like this, Parfit helped found population ethics, a branch of moral philosophy which considers how many people should exist and what we owe to people who are not yet born. We explore Parfit’s unsettling ideas about future generations – and why, decades later, we’re still grappling with the questions he raised. We also reflect on Parfit’s legacy and the key ingredients of his philosophy.