In 1973, astrophysicist and cosmologist Brandon Carter of Cambridge University proposed the Anthropic Principle, a theory which attempts to explain why our solar system seems to be finely tuned for intelligent life. That is, the universe gives the appearance of having been specifically designed to allow for human life on planet Earth. In their book The Anthropic Cosmological Principle, John Barrow and Frank Tipler define the most basic form of this concept:
"Weak Anthropic Principle (WAP): the observed values of all physical and cosmological quantities are not equally probable but they take on the values restricted by the requirement that there exist sites where carbon-based life can evolve and by the requirement that the Universe be old enough for it to have already done so."
Barrow and Tipler further proposed the Strong Anthropic Principle (SAP), which states that the Universe had to bring humanity into being: "The Universe must have those properties which allow life to develop within it at some stage in its history."
Some astronomers explain the fine-tuning of the universe as a result of chance. However, in "Home Alone in the Universe," Guillermo Gonzalez and Hugh Ross argue, "By insisting that the apparent fine-tuning of our solar system's parameters is just due to chance, rather than pointing towards a greater truth, astronomers have missed out on many discoveries."