The author is a professor of philosophy at Duke University who usually writes books for other philosophers and people who aspire to be philosophers. This one was written for a general audience. Maybe that’s why the book comes on so strong. Borrowing Nietzsche’s phrase, it’s philosophy with a hammer.
I assume Professor Rosenberg chose the title, but it’s a little misleading. Rosenberg derives his atheism from a more fundamental view called “scientism”. He defines that as the worldview according to which “the methods of science are the only reliable ways to secure knowledge of anything”. Unfortunately, there is no word that refers to someone who accepts scientism except “scientist” and you can definitely be a scientist without believing in scientism. Plus, a title like The Guide to Reality for People Who Accept Scientism isn’t exactly catchy. So “The Atheist’s Guide to Reality” it is.
One way Rosenberg explains scientism is to say that physics fixes all the facts (except, presumably, for the facts of logic or mathematics). Physics says that all events in the history of the universe, except some at the quantum level, are determined by previous events and the laws of nature. Furthermore, the Second Law of Thermodynamics (entropy ultimately increases in an isolated system) is the “driving force” behind evolution, which is the result of haphazard genetic mutation. Evolution gave us minds, but our minds are nothing more than the activity of our brains.
Rosenberg concludes that we don’t have free will, introspection is generally misleading and thoughts (whether conscious or unconscious) aren’t “about” anything (since what happens in a neuron can’t be “about” anything — it’s just a tiny input/output device). Furthermore, there are no purposes in nature, even in our minds, and there are no ethical facts. Morality is just another evolutionary adaptation. In addition, we can learn nothing from history or economics, since human culture is constantly evolving.
Rosenberg expresses his conclusions with an air of almost absolute certainty, which is odd for someone who believes in science (maybe it’s not so odd for someone who believes in scientism). For example, he says that “what we know about physical and biological science makes the existence of God less probable than the existence of Santa Claus”. Perhaps he’s being facetious in that passage, but many atheist or agnostic philosophers would agree that God’s existence is a metaphysical question beyond the reach of science. Natural processes don’t count for or against the supernatural. Besides which, there is no evidence at all for the existence of Santa Claus.