In this book, first published in 1638, Wilkins defended the Copernican and Galilean idea that the Earth is a planet by establishing analogies with the Moon. Following Galileo, Wilkins argued that if the Moon moves through the heavens and yet bears similarities with the Earth, such as mountains and oceans, then one should not discount the possibility that the Earth also is a heavenly body. Following Kepler, Wilkins also considered whether the Moon is inhabited and whether it might be possible to reach it by flight.