From “A.I. Has Grown Up and Left Home,” David Auerbach’s Nautilus article about how the field may be hamstrung by too much concern over how thinking “works,” a passage about the frustrations of the Cyc project:

“Unfortunately, not all facts are so clear-cut. Take the statement ‘Cats have four legs.’ Some cats have three legs, and perhaps there is some mutant cat with five legs out there. (And Cat Stevens only has two legs.) So Cyc needed a more complicated rule, like ‘Most cats have four legs, but some cats can have fewer due to injuries, and it’s not out of the realm of possibility that a cat could have more than four legs.’ Specifying both rules and their exceptions led to a snowballing programming burden.

After more than 25 years, Cyc now contains 5 million assertions. Lenat has said that 100 million would be required before Cyc would be able to reason like a human does. No significant applications of its knowledge base currently exist, but in a sign of the times, the project in recent years has begun developing a ‘Terrorist Knowledge Base.’ Lenat announced in 2003 that Cyc had ‘predicted’ the anthrax mail attacks six months before they had occurred. This feat is less impressive when you consider the other predictions Cyc had made, including the possibility that Al Qaeda might bomb the Hoover Dam using trained dolphins.”