The often repeated arguments about billions of galaxies full of stars suitable to hold in orbit planets much like our own make an eminent probabilistic sensebut they do not mean the inevitability of billion-fold replications of the Earth’s life. I am impressed less by the possibility of countless civilizations (including many vastly more sapient worlds than ours) sprinkled through the cosmic void than by the idea of the Earth being the only harbour of life in the universe. In any case, until we can get in touch with an extraterrestrial society, we should behave as if the existence of life confined to a single planet is the only reality. Then, if we are to remain and to prosper on this planet, we must reconcile our needs with biospheric limits. Our actions will continue changing the environment, but we must, not before too long, arrive at levels of interference which are compatible with long-term preservation of critical biospheric functions.