Computers are All Tactics and No Strategy

Chess computers don't have psychological faults, but they do have very distinct strengths and weaknesses, far more distinct than any equivalently strong human player would have. Today, they are so strong that most of their vulnerabilities have been steamrolled into irrelevancy by the sheer speed and depth of brute force search. They cannot play strategically, but they are too accurate tactically for a human to exploit those subtle weaknesses decisively. A tennis player with a 250-m.p.h. serve doesn't have to worry very much about having a weak backhand.

That was far from the case back in 1985. Tactical calculations were still a computer strength, but only shallow sequences three or four moves deep. This was more than enough to beat most amateurs consistently, although strong players became adept at setting tactical traps that were too deep for the computers to see. It seemed paradoxical that the machine's strength of flawless calculation was also a major weakness. The brute force "exhaustive search" method of checking every one of millions of positions also meant that the search tree couldn't reach very deep. If you could find a tactical threat that struck the decisive blow four moves (eight ply) away when the computer could only see three moves (six ply) deep, it wouldn't see it coming until it was too late. We call this the "horizon effect," exploiting that the machine can't see beyond its search "horizon."