> So let’s say you want to create a program that doesn’t have bugs. How should you do it? Fundamentally the only way to make sure a program will always do what you want is to understand everything it can do. But there’s a kind of paradox implicit in that: if you can really understand everything your program can do that basically means the program is doing something computationally reducible, and you probably in the end didn’t actually need to run the program with all its steps to get the result you wanted.

So... he has this "strange" idea of what makes a program useful... I wonder what does he think about a program that displays system time? Totally predictable! So, should it be just replaced by the output?

On a more serious note, at least from a perspective of a system programmer, vast majority of programs don't do (intentionally) this kind of nodeterministic stuff. They are more of a complex way of displaying information obtained by the sensors, or a way to cause the physical parts of the system to change their configuration. They are totally predictable, yet they still are absolutely necessary for the computer system to function.