“But where is the rationale—the raison d’etre—for our universe having these features? This is not a question borne of idle philosophizing about why certain details happen to be one way instead of another; the universe would be a vastly different place if the properties of the matter and force particles were even moderately changed.”

Greene explains that if the basic building blocks of the universe, such as the three families of matter particles and the four forces, were even slightly different than what they are, it would lead to massive changes in the laws of physics, so that the physical properties of the universe would be much different from observable reality. The question of why this should be so relates to the theme of The Human Need to Understand and hints at the limitations of current science.

“The discovery of the T.O.E.—the ultimate explanation of the universe at its most microscopic level, a theory that does not rely on any deeper explanation—would provide the firmest foundation on which to build our understanding of the world. Its discovery would mark a beginning, not an end. The ultimate theory would provide an unshakable pillar of coherence forever assuring us that the universe is a comprehensible place.”

The “theory of everything,” which Greene refers to here as the “T.O.E.,” is the search for a unifying theory to encompass all of physics, which relates thematically to The Unification of Physics. This is the most important theme in the book and was Greene’s motivation to write it. Thus, this theme recurs throughout. Greene argues that even if scientists find such a theory, it will not mark the end of discovery in science but a new beginning from which they can answer all the mysteries of the universe.