The thought of this book comes from an epiphany about eternity. Eternity is not a kind of "divine nature” but exists in our world as a temporal attribute of matter. This book expands the connotation of the concept of extension, starts from a series of concepts to discuss how matter is related to space-time, points out that space-time should be identified as extension, analyzes the relationship between time and reversibility, and then makes an overview about the Cosmos. From the view of the identity theory, there are common clues in such questions as what is the universe made of, whether the laws of physics at the singularity are invalid or not, the birth and destruction of our universe, multi-cosmology, and the reversibility of time.