If the Universe Is Teeming with Aliens ... WHERE IS EVERYBODY?: Seventy-Five Solutions to the Fermi Paradox and the Problem of Extraterrestrial Life

by Stephen Webb

The literature contains several types of anthropic reasoning, corresponding to several anthropic principles each with different shades of meaning. According to Carter, the weak anthropic principle (WAP) is that “what we can expect to observe must be restricted by the conditions necessary for our presence as observers.” The WAP seems almost tautologous. The strong anthropic principle (SAP), on the other hand, is more contentious: “the universe (and hence the fundamental parameters on which it depends) must be such as to admit the creation of observers within it at some stage.” Barrow and Tipler, in a classic book, also discuss the final anthropic principle (FAP), which they define as “intelligent information-processing must come into existence in the universe and, once it comes into existence, it will never die out”.