Type II civilizations dominate more than one solar system and/or are potentially capable of harnessing all of the power available from a single star (per Dyson). As a result, a civilisation that uses faster-than-light travel is likely to be classified as a Type II civilization. However, because an FTL drive needs to be able to generate enough energy to operate it, these civilizations will need to develop ways to capture energy from their environment. A Type I civilization that does not utilize FTL technology would be defined as a civilization that exists within our galaxy alone. It should be noted that there are various definitions for what constitutes an "advanced" or "civilized" society. Some scientists classify civilizations according to how long they exist before collapsing due to environmental damage or internal conflict. Others look at the complexity of their technology, while others still compare them with biological organisms. There is no single correct way to classify civilizations, but most researchers agree that Type II civilizations are the only ones likely to have evolved beyond Earth's own evolutionary stage. The first definition of a Type II civilization was proposed by American astronomer Carl Sagan. In his book The Demon-Haunted World: Science as a Candle in the Dark, he wrote that such a civilization would be capable of traveling through space and would probably have developed technologies enabling them to do so.