  Turing’s construction of a universal machine gives the most fundamental insight into computation: one machine can run any program whatsoever. No matter what computational tasks we may need to perform in the future, a single machine can perform them all. This is the insight that makes it feasible to build and sell computers. One computer can run any program. We don’t need to buy a new computer every time we have a new problem to solve. Of course, in the age of personal computers, this fact is such a basic assumption that it may be difficult to step back and appreciate it.