Rene' Descartes discussed what he called a reflex. A Reflex is the relation between a stimulus and a response. A stimulus is something we can sense. That is, something that can affect us.
Response is a behavior.
Separately, there is nothing special about just a stimulus and just a response. But when one reliably produces the other then we have something more. History of causal analysis Descartes then theorized 'why' the relation exists. His theory was good because it was something that could be observed in principle. That is, even though the technology at the time didn't allow him to observe the inside workings of nerves, it was understood that one day we can.