The text of a program does not show what is in memory, or what the devices attached to the machine are doing. You hope that if you run the program, in your head

The text of a program does not show what is in memory, or what the devices attached to the machine are doing. You hope that if you run the program, in your head, you can reliably guess -- but often, you make mistakes. Controlling "state" amounts to trying to force the world to match the text of your program as closely as possible, so you don't go wrong.