The nervous system

If I remember it right, for something to happen inside, something must happen on the outside.
Man, this stuff is kind of confusing!
But I love it.
So, if a person sees food, will the person always reach for it?
Every time they see food?
No, only when they are hungry....or something like that.
I also read about this guy named Rene Descartes with a little thingy on the first 'e' in 'Rene', who compared humans and animals with machines and robots.
And I read a lot about the nervous system, nerves, other nerves, more nerves, reactions, things like that.