Are we alone in the universe? Are there other beings on other worlds who gaze into the night sky and try to imagine us, as we try to imagine them? Those questions have been debated since antiquity, but it was during the Enlightenment that they particularly began to engage the interest of prominent scientists and thinkers. In this fascinating volume, Professor Michael Crowe offers the first in-depth study in English of the international debate that developed between 1750 and 1900 concerning the existence of extraterrestrial life, a problem that engaged an extraordinary variety of Western thinkers across the spectrum of intellectual endeavor. Astronomers such as Herschel, Bode, Lalande, and Flammarion all weighed in, along with French philosophers Rousseau and Voltaire, American patriot Thomas Paine, Scots churchman Thomas Chalmers, and a host of others. Professor Crowe gives them all their say, as they address the question as a point of science, as a problem of philosophy, as well as a religious issue. The book ends with the "discovery" by Schiaparelli of the canals of Mars, the expansion of the canal theory by the American astronomer Percival Lowell, and the culmination of the canal controversy with the demonstration of its illusory nature.