Knowledge, then, is less a canon than a consensus in a state of constant disruption. Part of the disruption has to do with error and its correction, but another part with simple newness—outright discoveries or new modes of classification and analysis, often enabled by technology.

ore commonly, however, changes in scientific facts reflect the way that science is done. Mr. Arbesman describes the "Decline Effect"—the tendency of an original scientific publication to present results that seem far more compelling than those of later studies. Such a tendency has been documented in the medical literature over the past decade by John Ioannidis, a researcher at Stanford, in areas as diverse as HIV therapy, angioplasty and stroke treatment. The cause of the decline may well be a potent combination of random chance (generating an excessively impressive result) and publication bias (leading positive results to get preferentially published). If shaky claims enter the realm of science too quickly, firmer ones often meet resistance. As Mr. Arbesman notes, scientists struggle to let go of long-held beliefs, something that Daniel Kahneman has described as "theory-induced blindness." Had the Austrian medical community in the 1840s accepted the controversial conclusions of Dr. Ignaz Semmelweis that physicians were responsible for the spread of childbed fever—and heeded his hand-washing recommendations—a devastating outbreak of the disease might have been averted.

Ranking The Battleground States

As a curious race we have always liked to know where we are, but it is now almost impossible not to know - our phones, computers and sat navs keep us continually co-ordinated, and through them we are involuntarily tracked ourselves. Once the preserve and privilege of the rich and influential, maps and accurate wayfinding have suddenly come to feel like a birthright

But these days we are all really at the centre of our maps, which is both a useful and egocentric thing. A thousand years ago Jerusalem stood at the centre of the Christian world view, or if you lived in China it was Youzhou. But now it is us, a throbbing green dot on our handhelds. We no longer travel from A to B but from Me to B, and we spread out maps on the floor or on our laps in a car only with wistful nostalgia.

It is still too early to say whether a lessening in our spatial ability and perspective, and our ability to remember landmarks, will decrease that area in our hippocampus that serves as the engine room for such skills, but it is highly likely. An examination of the brains of cab drivers has shown a great expansion in that area due, it is thought, to the retention of many miles of street plans.