Alex Rosenberg's The Atheist's Guide to Reality is a call to readers to embrace a scientistic view of knowledge. The word "scientistic" does not mean "scientific," rather it describes a view based on "scientism" which is the notion that science is the only reliable guide to truth about the world and human existence. On scientism, if a claim cannot be demonstrated empirically, using the tools of science, then it's not something that we can know, and in fact is not even something we should believe. In Rosenberg's telling, physics "fixes all the facts" about what is and what can be reasonably believed. This is also sometimes referred to as "physicalism."
Not all scientists are scientistic or "physicalists." Many of them hold that there are truths about the world that science is not equipped to discover, but Rosenberg thinks that scientists who believe this are practicing neither good science nor good epistemology.
Rosenberg is no dummy. He's the chairman of the philosophy department at Duke University and demonstrates in his book a considerable breadth of learning. He also strives to be rigorously consistent. Given his belief that physicalism is the only correct way to understand reality it follows for him that there is no God, no miracles, no soul or mind, no self, no real meaning or purpose to life, no meaning to history, no human rights or value, no objective moral duties - all of which leaves us with what he calls a "nice nihilism."
By the term "nice nihilism" Rosenberg means that nature has fortuitously evolved in us a tendency to treat each other well despite the fact that doing so is neither a moral duty nor in any way morally "right." That, for the one who embraces Rosenberg's scientism, is the only glimmer of light in an unrelentingly dark world and even this tiny glimmer is beset with problems. Here's one:
If our niceness is the product of impersonal undirected processes then it cannot have any moral purchase on us. That is, it can be neither right nor wrong to be "nice." Some people are and some aren't, and that's the end of the matter. Evolution has, after all, also developed behaviors that are not "nice." If we are solely the product of evolution then there's really no way to morally discriminate between "niceness" and "not nice," between what we call virtue and rape, torture, lying, racism, etc. All of these behaviors have evolved in our species just as niceness has, and we have no basis for saying that we are morally required to shun any of them. In other words, on scientism, there are no moral obligations and nothing which it is morally wrong to do.
Rosenberg frankly admits all this and insists that we need to face up to the fact that these are the consequences of adopting a scientistic worldview, and a scientistic worldview, in his mind, is the only intelligent option in a world in which there is no God.
I think he's right about this, actually, and argue in my novels In the Absence of God and Bridging the Abyss (see the links at the top of this page) that the sorts of consequences Rosenberg outlines in his book do indeed follow from an atheistic worldview. The atheist who lives as if none of these consequences exist is either in denial or he's living out an inauthentic, irrational delusion, most likely because he can't live consistently with the logical existential entailments of his naturalism.
A belief, though, that leads to conclusions one cannot live with stands in serious need of reexamination.