The people working on the Semantic Web greatly overestimate the value of deductive reasoning (a persistent theme in Artificial Intelligence projects generally.) The great popularizer of this error was Arthur Conan Doyle, whose Sherlock Holmes stories have done more damage to people's understanding of human intelligence than anyone other than Rene Descartes. Doyle has convinced generations of readers that what seriously smart people do when they think is to arrive at inevitable conclusions by linking antecedent facts. As Holmes famously put it "when you have eliminated the impossible, whatever remains, however improbable, must be the truth."
This sentiment is attractive precisely because it describes a world simpler than our own. In the real world, we are usually operating with partial, inconclusive or context-sensitive information. When we have to make a decision based on this information, we guess, extrapolate, intuit, we do what we did last time, we do what we think our friends would do or what Jesus or Joan Jett would have done, we do all of those things and more, but we almost never use actual deductive logic.
This illustrates a theory that I have held for a long time that we humans are no where near as logical as we think we are. Which raises the question why do we think we are so logical? Do we believe that if we just knew the right facts, and proper techniques, we could reason out how everything works and the universe would unfold a myriad of treasures at our feet? Or we believe that logic is the only tool for comprehending the universe and other people? I think at some level we do. I think logic has been hyped for centuries as the cure to all ills, and the above reference to Descartes illustrates how long the hype has been going on. While logic is obviously a useful tool, it falls far short of most this hype, and even in the arenas where it is useful it is often much more tedious and unwieldy than we want to believe.
The first place I heard the statement about eliminating the impossible was not from Sherlock Holmes, but from from one of his many intelecual heirs, that paragon of "space age" logic, Spock. But Spock and Holmes, are only two characters in a long line of "super logicians". Some other popular super logicians, James Bond, Dr. Hannibal Lecter, and Doc Savage. Frequently we accept the "super logician" myth while simultaneously mocking it. For example Buckaroo Banzai, or Austin Powers.
Truly great logicians respect the limits of deductive logic. For example, Einstein said "Where math describes reality it is not precise, and where it is precise, it does not describe reality."
Obviously, I'm not advocating abandoning logic completely. Logic is a very powerfull tool, but it is probably one of the most misused intellectual tools, especially when trying to figure out how and why people do what they do. The biggest abuse is when we try to figure out why not everyone else thinks as "logically" as we do.
