"Porter was interested in how companies succeed. The scholar who in some respects became his successor, Clayton M. Christensen, entered a doctoral program at the Harvard Business School in 1989 and joined the faculty in 1992. Christensen was interested in why companies fail. In his 1997 book, "The Innovator's Dilemma," he argued that, very often, it isn't because their executives made bad decisions but because they made good decisions, the same kind of good decisions that had made those companies successful for decades. (The "innovator's dilemma" is that "doing the right thing is the wrong thing.") As Christensen saw it, the problem was the velocity of history, and it wasn't so much a problem as a missed opportunity, like a plane that takes off without you, except that you didn't even know there was a plane, and had wandered onto the airfield, which you thought was a meadow, and the plane ran you over during takeoff. Manufacturers of mainframe computers made good decisions about making and selling mainframe computers and devising important refinements to them in their R. & D. departments—"sustaining innovations," Christensen called them—but, busy pleasing their mainframe customers, one tinker at a time, they missed what an entirely untapped customer wanted, personal computers, the market for which was created by what Christensen called "disruptive innovation": the selling of a cheaper, poorer-quality product that initially reaches less profitable customers but eventually takes over and devours an entire industry."