Abstract: The Gaia theory of James Lovelock proposes that the Earth is a self-regulating system, or super-organism, maintaining conditions hospitable to contemporary planetary biota. Objections to this theory, concerning its alleged untestability and circularity, are considered and countered. Favourable evidence includes Lovelock’s daisyworld model of a planet regulating its own temperatures and thus maintaining homeostasis, and his discoveries of actual regulatory mechanisms such as the biological generation of dimethyl sulphide, which removes sulphur from the oceans and seeds clouds whose albedo reduces solar radiation (a negative feedback mechanism). After some decades of scepticism, sections of the scientific community have partially endorsed Gaia theory, accepting that the Earth system behaves as if self-regulating. Whether or not this theory is acceptable in full, it has drawn attention to the need for preserving planetary biological cycles and for the planetary dimension to be incorporated in ethical decision-making, and thus for a planetary ethic.
Comment: This interdisciplinary survey of the Gaia hypothesis, its critics and its supporters, could be used in Philosophy of Science or Philosophy of Biology classes to clarify the concept of Gaia, which is often presented too vaguely by those who have not considered issues such as whether this hypothesis is falsifiable or not; it could also be used in Ethics classes because of its section on Gaian ethics. We show how Lovelock has devised indirect ways of testing this hypothesis (or better, the Gaia theory), how a critic (Kirchner) has presented it as either falsifiable but unsurprising or unfalsifiable and thus useless, and how a supporter, Tim Lenton has sought to explain how it can be reconciled with Darwinian evolution. Finally we show how elements of the theory have been endorsed by a scientific conference, but other aspects, such as the purposiveness of Gaia, were not endorsed.