Comment by mfenniak

Not really.

If a program is built with strong software architecture, then a lot of it will fit that definition. As an analogy, electricity in your home is delivered by electrical outlets that are standardized -- you can have high confidence that when you buy a new electrical appliance, it can plug into those outlets and work. But someone had to design that standard and apply it universally to the outlets and the appliances. Software architecture within a program is about creating those standards on how things work and applying them universally. If you do this well, then yes, you can have a lot of code that is testable and verifiable.

But you'll always have side-effects. Programs do things -- they create files, they open network connections, they communicate with other programs, they display things on the screen. Some of those side-effects create "state" -- once a file is created, it's still present. These things are much harder to test because they're not just a function with an input and an output -- their behavior changes between the first run and the second run.