Dilemma of a scientist looking at a horse, plow and a plow-boy

Which way does the causality run? Is it the plow pushing the horse or is it the horse pulling the plow? And what if it is a spurious correlation, and it is the person holding the rein that causes the movement of both horse as well as the plow? Or is the plow-boy an antecedent variable that affects the horse which is an independent variable that causes the movement of the plow which is the dependent variable? Or is the plow-boy genuine independent variable that causes both horse as well as plow to move and both horse as well as plow are dependent variables that just covary?

And what if it is actually government subsidies that make plow-boy cause all of the above? Well, that would be a question of level of abstraction, I guess.