NASA has spent decades, and billions of dollars, looking for life in our solar system. Just this month they issued a major press release announcing that they have found organic matter on Mars. It is far from clear whether they have found life, however. This is partly because what they found is a molecular ‘sign’ of life, rather than potential life forms. But it is also because there is no agreed-upon understanding of what counts as life in the first place. Here we run into an unresolved debate that goes back at least 2000 years: What is life? Today this debate has arrived at an impasse, with over 100 proposed definitions of life and little movement towards consensus within disciplines, let alone across them. This has led some philosophers and scientists to dismiss the whole project of defining life as misguided or hopeless, and to suggest that we give up and find something else to argue about.