by Explanation — not a question of simply adding nodes to a causal model If we do not fully explain by adding more variables, how do we explain? Mechanisms explain because they embody an invariant property. The first mechanism, linking the gas pedal to the rotating drivetrain, is combustion: The second mechanism, linking the rotating drivetrain to acceleration, is the relationship of torque to force. Combustion is a high energy-initiated, exothermic (heat-generating) chemical reaction between a compound such as a hydrocarbon and an oxidant such as oxygen. The heat generated by combustion increases pressure in a sealed cylinder and impels a piston. A similarly brief description could be given of the relationship between torque, force, and acceleration. The
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Explanation — not a question of simply adding nodes to a causal model
If we do not fully explain by adding more variables, how do we explain? Mechanisms explain because they embody an invariant property. The first mechanism, linking the gas pedal to the rotating drivetrain, is combustion: The second mechanism, linking the rotating drivetrain to acceleration, is the relationship of torque to force. Combustion is a high energy-initiated, exothermic (heat-generating) chemical reaction between a compound such as a hydrocarbon and an oxidant such as oxygen. The heat generated by combustion increases pressure in a sealed cylinder and impels a piston. A similarly brief description could be given of the relationship between torque, force, and acceleration. The key point is this: Combustion is not a variable. In the proper circumstances -the combination of specific compounds and oxidizing agents, with a high energy initiation to overcome the stability of dioxygen molecules-combustion occurs with law-like regularity. That regularity can in turn be explained by more fine-grained physical processes at the subatomic level.
By saying that a mechanism like combustion is invariant, not a variable, I am stating that it cannot be directly manipulated; one cannot intervene to turn combustion off. We can block the mechanism of combustion from working by intervening on ancestral nodes. We can remove all fuel from the car or disable the electrical system; in either case, depressing the gas pedal will not cause the car to accelerate any longer because combustion cannot take place. But if gas is present and the electrical system is functioning, we cannot intervene to set combustion to the value “off”. If a fuel-air mixture is combined with a functioning ignition system, combustion occurs, and so combustion should not be represented by a node in the causal graph upon which we could intervene. Mechanisms embody invariant causal principles and, in particular instantiations, they generate observed correlations. Mechanisms are thus distinct from causal pathways; they explain the location of the directed edges (arrowheads). It is the invariant causal principle combustion that explains why (given a few more mechanical details) depressing the gas pedal makes the drivetrain rotate. Mechanisms explain the relationship between variables because they are not variables.