The promise of the Web is the world at our fingertips. Thousands of hours of research can be yours with a few adept keystrokes. Entertainment options are unlimited and at your command. Yet something is not right. Instead of intellectually flourishing, we have twitchy distractedness. Where is the contemplation, the deep thinking?

In this age of "content" abundance, why do we feel so dumb? In "The Shallows: What the Internet Is Doing to Our Brains," tech journalist Nicholas Carr attempts an answer: It's the medium, stupid. We've focused on what we can find/read/see -- that legendary Velvet Underground performance, the side effects of modafinil -- and we've lost sight of the real problem. Channeling Marshall McLuhan, Carr maintains that we are "too busy being dazzled or disturbed by the programming to notice what's going on in our heads." Our reading and thinking have adapted to the computer screen with consequences good and ill.

Carr presents a damning case against a life jacked into the Net, including the startling revelation that prolonged usage alters our brain physiology. Because the human brain is capable of incredible change even into adulthood (neuroscientists call it plasticity), we adapt readily to new tools. The "single most mind-altering" tool is the Internet, as it "delivers precisely the kind of sensory and cognitive stimuli -- repetitive, intensive, interactive, addictive -- that have been shown to result in strong and rapid alterations in brain circuits and functions."

In other words, the Web is changing our brains!

The implications of our Web dependence go beyond our individual brains, though, and into the heart of our culture. Skimming and scanning replace deep reading. As reading changes, so does writing. Bold headlines and short paragraphs make for easy power browsing. Carefully drawn out arguments, bolstered by evidence, do not. It is as if we are reversing "the early trajectory of civilization" and "evolving from being cultivators of personal knowledge to being hunters and gatherers in the electronic data forest."

At times, Carr throws things at the wall to see what sticks, masking contradictory, or at least paradoxical, claims. Sometimes the Web harms the brain because it overtaxes it, sometimes because it makes things too easy. Both may be true but they are very different problems. Issues of content and medium are blurred. Isn't a Web page with multiple elements on it -- video, hypertext, annoying Flash-based ad -- a failure of presentation and design rather than the medium itself?

Missing most in this book is a sustained defense of what is being lost. Those who have grown up in the digital age may not be easily convinced of the importance of deep thinking and the rich life of the mind.

The achievement of "The Shallows" isn't that it persuades you to give up the Web. Instead, it encourages you to be mindful of your screen time and remember that the Internet is the tool, not you.

Martin Schmutterer is a writer and bookseller in St. Paul.