Other authors have pointed out that the explanandum of life is itself up for debate (Tsokolov 2009, Mix 2020, Parke 2020). According to Emily Parke, some accept life as applying to individuals, whereas other definitions apply to collectives first (including entire planets) and individuals derivatively. Relatedly, most believe life is some kind of entity rather than some kind of relation or process (but see Nicholson and Dupré 2018). Parke also points out that some definitions seek a material basis, perhaps limiting life’s substrate to the biochemistry we know on Earth, while others are functional. Sagan famously worried about biochemical definitions because they were prone to ‘Earth Chauvinism’ for privileging our own biochemistry (1970). Other authors take our biochemistry to be independently justified as universal (Pace 2001, Benner et al. 2004, but see Bains 2004). Finally, Parke distinguishes between those that seek clean boundaries and those that accept the possibility of a continuum of ‘lifelikeness.’