Judea Pearl's perspective on this question is that causality is best viewed as an intervention originating from outside the system in question, and not as a mode of behavior within that system. In this view, causality is related to counterfactual queries we might ask about the system, e.g., if an intervention had forced situation X, would Y have occurred? Because the intervening agent always stands outside the system, causality is always relative to where we draw the boundary around the system, and thus is not a property of reality.