Exhaustively and exhaustingly thorough, this book surveys a huge number of writers, famous and obscure, well-informed and otherwise, who wrote about extraterrestrial life between the mid-18th century and 1900, with an extension covering the Martian canal debate up to around Lowell's death in 1916. Perhaps suprisingly, most people who wrote on the subject during the period where in favour not only of extraterrestrial life in general, but of the existence of intelligent life on our neighbouring planets; indeed often of the inhabitation of the Moon, the Sun, and even comets! The canal-builder Martian hypothesis should, Crowe stresses, not be seen so much as something new as as a last attempt to save at least one of our solar system neighbours as the abode of life in the face of mounting evidence of their uninhabitability.

Another thing Crowe stresses is that writers' views on extraterrestrials, whether pro or con, were often closely intertwined with their religious beliefs. Natural theologians saw the assumed existence of extraterrestrials as further proof of God's omnipotence and benevolence, while some Christians found it impossible to square Christ's redemptive mission to Earth with the existence of other mankinds on other planets. But there's little obvious pattern; Catholics, Anglicans, deists, materialists, and others are all found on both sides of the debate. He's coy about his own religious affiliations, but the last words of the conclusion would seem to confirm him as a theist of some sort.

The book focuses mostly on Great Britain, the US, France, and the German-speaking world, with occasional forays into the rest of Europe (and one to Canada). What if anything non-Westerners wrote or thought on the subject is not dealt with - which may be a good thing, given the size of the book as it is.