Ken Thompson's Reflections on Trusting Trust

Ken Thompson's point is philosophical, more than practical. It tells us something about the nature of what we're doing, rather than having some immediate practical lesson like "fire anyone devious who works for your software company" or "always use two compilers".

The system is too complicated for us to understand from the ground up how it all works, so we trust some aspects of the system implicitly. Thompson's example is the compiler, but it could as easily be the CPU or any other sufficiently complicated component. Probably we have long since passed the point of no return. All we can do is be aware of the trap we have set ourselves.