“There is a simple reason why you should commit yourself to writing programs that are free of errors from the very start. It is that you will never be able to establish that a program has no errors in it by testing. Since there is no way to be certain that you have found the last error, your real opportunity to gain confidence in a program is to never find the first error. The ultimate faith you can have in one of your programs is in the thought process that created it. With every error you find in testing and use, that faith is undermined.”