Most controversial has been the case of Gaia. Gaia is a term from Greek mythology; she is a personification of the planet Earth. In 1979, James Lovelock, revived the concept in his book, Gaia: A New Look at Life on Earth. In his view, the Earth-wide set of interlocking ecosystems could be viewed as a single entity. One insight of Lovelock’s was already mentioned in the previous section: planet-wide interactions are the scale that matters in detecting life elsewhere. Lovelock’s book sparked controversy centered around the plausibility of his model of the Earth as a self-regulating homeostatic system. In the view of many at the time, it was an inaccurate description: Earth could not evolve in principle,  and the subsequent ontological move of granting Earth the status of life was unmotivated (Doolittle 1981). Recent attempts to revitalize the notion of Gaia on a more theoretic footing involve both abiotic and biotic regulatory mechanisms and natural selection acting at the level of clades (Lenton et al. 2018, Doolittle 2019). Regardless of current attempts at a theoretical justification, the thought of Earth as a living entity motivated many in the environmental movement and the idea remains a common reference.