Philosophy
The Metaphysics of Causation: Counterfactual vs. Process Theories
Quick fact
Counterfactual theories define causation by what would have happened if the cause had not occurred, while process theories define it by physical links that transmit energy or information—often leading to opposite verdicts on tricky cases like omissions.
Why this is interesting
We say a falling rock broke the window, but how do we know the rock, rather than just the window's own fragility, caused the break? Philosophers have two very different ways of answering that question.
Read the full explanation
Understanding The Metaphysics of Causation: Counterfactual vs. Process Theories
Let's start with a simple example. Suppose you throw a ball through a window, and the glass shatters. What made the glass break? Most of us would say: the ball hitting the window. But to a philosopher, that 'causing' is surprisingly hard to pin down. One natural idea is the counterfactual approach: if the ball had not hit the window, the glass would not have broken. That seems to capture our intuition that the ball was the cause because it made a difference. But consider a case where two snipers simultaneously shoot a victim; each bullet alone would be fatal. If you ask 'would the victim have died if sniper A hadn't fired?' the answer is yes—because sniper B's bullet would still have killed him. So the counterfactual test says A's shot was not a cause, which seems odd. This reveals a problem: the counterfactual account focuses on dependence—whether the effect depends on the cause. A different, process-based view starts with the idea of an actual physical connection. It looks at the world: a bullet travels from the gun to the victim, transferring energy and momentum. A process theory says a causal connection happens when there is a genuine physical process—like a moving ball or a propagating wave—that transmits a conserved quantity like momentum from one event to another. In the ball-window case, the ball carries momentum to the glass, so there's a clear causal process. In the sniper case, both bullets are separate processes, and each is a cause of the death, even if outcome would have occurred anyway. So the two theories highlight different aspects of causation: one emphasizes difference-making, the other emphasizes physical mechanism. Neither perfectly matches all our intuitions, and philosophers continue to debate which is more fundamental.
A deeper explanation
The counterfactual theory, famously championed by David Lewis, analyzes causation in terms of counterfactual dependence: an event C causes an effect E if, had C not occurred, E would not have occurred. This uses the framework of possible worlds: we consider the possible world most similar to our own where C does not occur, and see whether E also fails to occur. Crucially, Lewis used a non-transitive notion of causation, defining it as a chain of counterfactual dependencies, which helps handle some preemption cases but still struggles with overdetermination and with cases of causal preemption where the effect would have occurred anyway. The theory captures the important idea that causes are difference-makers, and it fits well with our ordinary reasoning about what would have happened under different circumstances. But it has limitations: it cannot easily handle cases where effects are overdetermined, and it seems to make causation mind-dependent because it relies on similarity relations between worlds. Process theories, on the other hand, especially as developed by Wesley Salmon and Phil Dowe, focus on actual physical connections. A causal process is a sequence of events that has the ability to transmit a conserved quantity—such as momentum, energy, or charge—along a continuous path. A causal interaction occurs when two such processes intersect and exchange a conserved quantity. For example, a baseball's flight is a causal process; its impact on a window is a causal interaction that changes both the ball's and the window's momentum. This theory gives a more objective, physical basis for causation, grounding it in the laws of physics. It avoids the problem of overdetermination because each process is a genuine cause regardless of counterfactual dependence. However, it fails to capture cases of omission (e.g., a gardener's failure to water a plant causes it to wilt) because there is no positive process. It also struggles with mental causation, since mental events do not obviously involve physical conserved quantities. Ultimately, the choice between counterfactual and process theories reflects a deeper question: is causation a matter of what would have happened, or is it a matter of what actually happens in the physical world? Counterfactual theories emphasize the modal nature of causation, while process theories emphasize its physical nature. A full understanding of causation may require both: the counterfactual theory captures the idea of causal relevance, and the process theory captures the physical mechanism.