There's no writer more lucid than Jane Jacobs, nobody better at using wide-open eyes and clean courtly prose to decipher the changing world around us. So it's heartening that even in her late 80s, in a book whose title hints at common sense giving way to broad-brush gloom, she studies the economy by lifting up a chair.

"Turning a chair upside down for a clue to its provenance, I found a label reporting 'Made in Canada,' " Jacobs writes of a visit to a Toronto suburb where jobs are increasing even though conventional economic wisdom says they shouldn't. "A prowl among offices in industrial parks might suggest further grist for import replacing and innovation."

That sort of dogma-free inquisitiveness sharpens everything that Jacobs has written since 1961's "The Death and Life of Great American Cities." In her new book, "Dark Age Ahead," Jacobs visits her old thematic haunts -- the way cities work, the way economies work, the shakiness of too much "accepted" wisdom -- to bring back an ominous new message: Lazy thinking and a lack of accountability could combine to unhinge many of the advances that fuel our modern life.

"A culture is unsalvageable if stabilizing forces themselves become ruined and irrelevant," Jacobs writes, arguing that a simple societal trait such as the urban necessity of locked doors is ominous because "the collapse of one sustaining cultural institution enfeebles others, makes it more likely that others will give way ... until finally the whole enfeebled, intractable contraption collapses."

Rather than dwell on obvious problems, such as violent crime or environmental ills, Jacobs pulls out "less recognized ... insidiously decaying" aspects of contemporary culture such as tax policies that starve the public realm; the lack of shame and self-policing in the financial sectors of the economy; and science that lets theory cloud reality.

The impromptu chair survey comes in the midst of the chapter "Science Abandoned," where Jacobs, a Toronto resident, uses Canadian national economic surveys as an example of analytical blindness with potentially dangerous outcomes. In this case, Jacobs exposes the disconnect between a national policy that clings to the importance of imports from the United States, and local entrepreneurs who are finding the profits in building a better product close to home. If those businesses are not nurtured, given better transit and stable schools, this potential renaissance might instead die off.

Another example draws on work done by former San Francisco planning director Allan Jacobs (no relation) and his wife, UC Berkeley urban design lecturer Elizabeth MacDonald. Jane Jacobs mentions approvingly the pair's "The Boulevard Book" from 2002: It argues the virtue of roadways that place neighborhood traffic in outside lanes and "express" traffic in the middle with landscaped medians in between as a way to create attractive community centerpieces rather than off-putting expressways. More to Jane Jacobs' point, the pair shows that while such roadways fell out of favor after 1930 because of safety concerns, no existing studies show any proof that the danger exists. It's an example of more bad science -- at the expense of livable neighborhoods.

"Why can't we continue surviving bad science?" Jacobs asks. The answer comes with disarming force: "Bad farming practices in the past resulted in poor yields and depleted soils; today, chemical fertilizers, toxic sprays, and doses of hormones and antibiotics to livestock endanger not only soils but also the health of farmers, farm laborers, consumers and the environment. Modern life has raised the ante of knowledge required in everything from science to democratic participation. Failures were always stultifying; now they can be devastating."

Where the book's premise seems more heartfelt than convincing is the notion that all this translates to the warning signs of a Dark Age during which modern civilization -- the whole shebang -- could somehow unwind and regress.

"The Death and Life of Great American Cities" remains Jacobs' most influential book; in it, she used her ground-level experience of Greenwich Village and Boston's North End to put the lie to urban renewal. Rather than trust theorists who saw orderly development as a virtue, she showed that the cacophony of neighborhood life is a sign of health rather than blight.

Even now the particulars in "Death and Life" seem fresh -- discoveries made by somebody taking the trouble to look at real life. But with "Dark Age Ahead," Jacobs is extrapolating from decades' worth of thinking. The difference is enormous because a reader must accept each conclusion on the way to the next one. If you don't agree, say, that the scuttling of urban rail systems after World War I is analogous to China's abandonment of its trading fleets in the 15th century, then every rhetorical thrust that follows becomes suspect.

Yet it's a tribute to Jacobs that her observations still resonate, succinct yet dead on. That's why "Dark Age Ahead" is a treat to read for the way it snaps our perceptions into focus, even if you don't buy into the overarching theme.

For instance, she suggests that a financial scandal à la Enron isn't simply an example of corporate greed but a warning that "when a profession with responsibilities like that [accounting] goes rotten, it is a cultural and economic nightmare. ... In olden times it would have been said that transgressing members of the profession had sold their souls to the devil."

Perhaps this is a sign of cracks in civilization. Perhaps it simply reflects the value-free cynicism of our age. And perhaps, just perhaps, the two are not that far apart.