Who can be wise, amazed, temperate and furious,/Loyal and neutral, in a moment? No man:/The expedition of my violent love/Outrun the pauser, reason.

--Macbeth, Act 2, Scene 3

DECISIONS--EVEN snap decisions--must come from somewhere. Some part of the brain must select, plan, and execute every one of our actions, even if the entire process unfolds in milliseconds, too fleeting for scrutiny by the conscious mind. At the same time, other parts of the brain must sift the torrents of information that inundate our senses, choosing which of the various streams to direct to our attention and which to divert. Only rarely is there time to ponder. More often we react in an instant: we like a new food or hate it; we smile at a stranger or look away; laugh at a joke or roll our eyes. All of this happens in the span of a heartbeat. And most of us, for the most part, have as little insight into our thoughts in that one beat as Macbeth claimed to have had when he murdered King Duncan's chamberlains.

Malcolm Gladwell, a staff writer for the New Yorker and the author of a previous bestselling book, The Tipping Point, about how "little things can make a big difference," wants to know how this works. He calls this kind of cognition Blink--as in the blink of an eye. What is more, he suspects that "blink," or, as his subtitle puts it, "thinking without thinking," might sometimes be more accurate and more powerful than reason, even if at other times it leads us astray. His ultimate aim is to discover what makes for good instincts and, conversely, why our instincts are sometimes wildly off the mark.

If we can understand that, Gladwell believes that we can actually tame and educate the unconscious mind. If he is right, that would be no mean feat.

GLADWELL BEGINS by relating an episode notorious in art circles. In 1984, the J. Paul Getty Museum purchased a kouros, a kind of archaic Greek statue, at a price of just under $10 million. Naturally, the Getty was convinced that the artifact was authentic: it seemed to have a bona-fide provenance, and scientific tests suggested that the marble was appropriately aged. And yet, strangely enough, every art historian who looked at the kouros sensed instantly, viscerally, that it was a fake.

In the end, of course, the art historians were proved right; under closer scrutiny, the Getty's case for the statue withered. What Gladwell wants to understand is why the experts were right--how they could apprehend in a single glance what months of research by lawyers and chemists had failed to prove.

Curiously, when he took the obvious approach and simply asked the art historians, Gladwell got nowhere. One reported that he was overcome with nausea at the sight of the object, while another said that he felt there was a glass wall between him and the statue. In other words, they knew, but they did not know how they knew. Even more curiously, Gladwell found the same was true of experts in other fields when placed in analogous situations.

Most curious of all is the fact that each of us seems to have extremely poor insight into the subject we know best--namely, ourselves. Especially when we act quickly or spontaneously, our responses are shaped less by our conscious thoughts than by information that happens to be percolating through the subconscious mind. What accounts for this?

Gladwell finds some leads in the work of social psychologists who study behavior in controlled settings. The simple act of solving a word puzzle with items like worried, Florida, and bingo can cause us to do an impression of old age, walking more slowly without noticing it. More ominously, researchers have found that people of all races are quicker to form implicit associations between "black" and "bad" than between "black" and "good"--even people who profess to be completely free of conscious racism or prejudice. To Gladwell, such findings underscore the idea that much of our thought takes place behind a locked door, and what goes on behind that locked door can shape our responses in unknown ways.

TO TAME our instincts, then, we need to crack open the lock. The key seems to be something Gladwell calls "thin-slicing": the ability to extract, in an instant, the handful of features in any situation that are crucial to making a judgment.

A paradigm of thin-slicing is the work of the psychologist John Gottman, who has developed a startlingly accurate method of predicting the success or failure of marital relationships by picking out certain telltale signs. Gottman has found, for example, that a contemptuous remark or a roll of the eyes--the sort of thing that many of us would be inclined to ignore--can spell disaster for a marriage. He knows because he has thousands of observations and years of data to prove it; his statistics show that fifteen minutes of conversation are enough to determine, four times out of five, whether a couple will still be married fifteen years later.

In some sense, we thin-slice every day. Our first impressions, our spontaneous likes and dislikes, are all examples of snap judgments based on thin slices of information. And, as it turns out, those judgments can be remarkably accurate. Untrained observers can predict how students will evaluate a teacher's effectiveness just by observing a few seconds of a lecture.

Of course, sometimes we guess wrong. Doctors in emergency rooms, Gladwell tells us, are often distracted by risk factors that may be relevant in the long term, but not in the immediacy of the moment--causing them to misjudge, for example, the chances that a patient has suffered a heart attack. Police officers, caught up in the heat of a high-speed chase or on patrol in a gritty and crime-ridden neighborhood, can become trapped by catastrophic errors. A tragic example cited by Gladwell is the 1999 shooting of Amadou Diallo, the Guinean immigrant whose wallet was mistaken for a gun late at night in the Bronx.

But with training, Gladwell suggests, we can learn to attend to information that is truly valuable and to discard the irrelevant and the misleading. We can become attuned to thinner slices, the ones with ever greater predictive value. For example, the average person is adept at reading facial expressions for clues about what others are thinking. Those trained to thinslice, however, can detect "microexpressions," ephemeral twitches of the facial muscles that cannot readily be faked or suppressed--revealing in that instant whether someone is lying or bluffing, secretly happy or full of disdain.

And thin-slicing is good for more than parlor tricks. By following a simple decision tree, doctors can focus on the information that is most crucial to a patient's immediate fate. Patrolmen can learn to keep their gut reactions in check, thin-slicing to gauge better whether a suspect is armed or unarmed, dangerous or harmless.

IN six chapters, Gladwell lays out for the reader a trove of tantalizing examples of "thinking without thinking." What do they all have in common, and what can we learn from them?

Alas, the problem with Blink is that Gladwell does not seem to know--or if he does, he is not telling. As a result, the book is dissatisfying, replete with unanswered questions, terminological confusion, and garden paths wandered down to no purpose or outlet. The idea seems to be that a diverse set of phenomena--from the gut feelings of experts to the biases of everyday life--are instances of the same kind of thought; unfortunately, though, Gladwell does not go much deeper than that, papering over the resulting deficit by the promiscuous use of false analogies and buzzwords. Is the adrenaline rush of a policeman chasing a suspect fundamentally similar to the profound social impairment of an autistic child? Probably not, especially from a neuroscientific point of view; nevertheless, Gladwell refers to them both as instances of "mindblindness."

And just what, according to Gladwell, is "thin-slicing"? As he defines it, it is the ability to pick out the few crucial facts that separate a good judgment from a bad one. But then, how is it that the experts who spotted the counterfeit kouros were thin-slicing? They seemed to respond to the statue as a whole, without being able to pinpoint any obvious giveaways.

Indeed, much more can be said than this. Neuroscience teaches us that the assessment of objects by experts is more like recognizing faces than recognizing facial expressions. Specifically, when we look at faces, we rely on a part of the brain called the fusiform gyrus, tucked into the hollow of the temporal bone underneath the ear. The fusiform gyrus is exquisitely sensitive to faces as wholes, allowing us to tell apart people who may look very similar to one another--and conversely, allowing us to identify the same person regardless of age or cosmetic change. On the other hand, most of us have no need to discriminate carefully among other objects (like pieces of marble), so we use a cruder visual processing area elsewhere in the brain.

What is different about experts is that they use the fusiform gyrus not only for faces, but also for the objects of their expertise. To an expert in archaic sculpture, then, looking at a take kouros must be like looking at a person with a wig and false teeth: something is amiss, even if we cannot immediately identify what it is.

Blink never bothers to venture this kind of explanation, or indeed to probe very deeply into any of the phenomena it describes. What accounts for the experts' revulsion at the kouros? What is it about the brain that is responsible for "gut" feelings? Gladwell notes that our bodies often respond before our brains catch up; gamblers sweat before they become consciously aware of risky bets, and sometimes we smile or frown before we feel elated or depressed. Something, surely, connects all of these observations. Is it that rapid judgments are funneled through the ancient provinces of the nervous system--the parts responsible for increasing our heart rates and turning our stomachs? Whatever the answer, the question itself has eluded Gladwell entirely.

IF GLADWELL does not dig anywhere near deeply enough, he errs in a different direction by concocting bizarrely complicated interpretations of simple phenomena. It is possible, as he suggests, that the policemen who shot Amadou Diallo were rendered temporarily blind--by the fear of a gun, perhaps, or by their implicit biases--to the facial expressions that would have otherwise suggested his innocence. It is also possible that they had trouble seeing in the dark. And if emergency-room doctors have trouble diagnosing heart attacks, is it really because the more information they have, the worse off they are? More likely, the problem is that ER doctors who dispatch patients to the cardiac-care unit rarely find out which of them actually had suffered a heart attack and which had not. In other words, the trouble is not too much information, but a lack of it: they never get the feedback necessary to refine their personal decision trees.

In his introduction to Blink, Gladwell promises to change the way we think about thinking. On closer inspection, the book turns out to be little more than an amalgam of interesting observations and tiresome political bromides, like the need to root out our supposedly pernicious unconscious prejudices by surrounding ourselves with diverse-looking people. And that is a shame, because the subject is a fascinating one; there is much still to be said about the hidden machinations of the human mind.