I used to pass by a large computer system with the feeling that it represented the summed-up knowledge of human beings. It reassured me to think of all those programs as a kind of library in which our understanding of the world was recorded in intricate and exquisite detail. I managed to hold onto this comforting belief even in the face of 20 years in the programming business, where I learned from the beginning what a hard time we programmers have in maintaining our own code, let alone understanding programs written and modified over years by untold numbers of other programmers. Programmers come and go; the core group that once understood the issues has written its code and moved on; new programmers have come, left their bit of understanding in the code and moved on in turn. Eventually, no one individual or group knows the full range of the problem behind the program, the solutions we chose, the ones we rejected and why.

Over time, the only representation of the original knowledge becomes the code itself, which by now is something we can run but not exactly understand. It has become a process, something we can operate but no longer rethink deeply. Even if you have the source code in front of you, there are limits to what a human reader can absorb from thousands of lines of text designed primarily to function, not to convey meaning. When knowledge passes into code, it changes state; like water turned to ice, it becomes a new thing, with new properties. We use it; but in a human sense we no longer know it.

The Year 2000 problem is an example on a vast scale of knowledge disappearing into code. And the soon-to-fail national air-traffic control system is but one stark instance of how computerized expertise can be lost. In March, the New York Times reported that IBM had told the Federal Aviation Administration that, come the millennium, the existing system would stop functioning reliably. IBM's advice was to completely replace the system because, they said, there was "no one left who understands the inner workings of the host computer."

No one left who understands. Air-traffic control systems, bookkeeping, drafting, circuit design, spelling, differential equations, assembly lines, ordering systems, network object communications, rocket launchers, atom-bomb silos, electric generators, operating systems, fuel injectors, CAT scans, air conditioners -- an exploding list of subjects, objects and processes rushing into code, which eventually will be left running without anyone left who understands them. A world full of things like mainframe computers, which we can use or throw away, with little choice in between. A world floating atop a sea of programs we've come to rely on but no longer truly understand or control. Code and forget; code and forget: programming as a collective exercise in incremental forgetting.