How do we learn about causation? David Hume thought that we do so from observing two types of events always co-occurring. Whenever the first event occurs, the second event follows, and from this we infer that the first event causes the second. Causation is then marked by regular, law-like behaviour.
Hume’s account has inspired the principle that same cause always gives same effect. Statistical methods are premised upon this principle, but also our scientific experiments are supposed to be reproducible: the same result should occur even if the same experiment is performed elsewhere and by other scientists. The strongest proof of causation would then be to have our causal expectations constantly and repeatedly confirmed. But is it really so?
I know that pressing the light switch causes the lamp to light and I can confirm this by doing it over and over. But is this what gives me the best causal knowledge? Sometimes this effect doesn’t follow: if the wiring is damaged, the socket is unplugged, there’s a short circuit, the lamp bulb is broken, burned out or not properly inserted, et cetera. Each of these situations teaches me something more about how pressing the button causes the lamp to light.
Now think of a situation where we don’t yet know how the cause produces the effect. Then it’s essential not to only focus on the successful cases, but to learn all we can from cases where causation breaks down. If a new drug is put on the market only because it’s repeatedly confirmed to produce the predicted effect, this can’t be the full causal story. The drug does nothing on its own but only in causal interaction with the bio-chemical processes in the human body. When we learn about some unpredicted effect of the drug, we also learn more about how the drug works causally.
Causation is essentially context-dependent. Different causal factors are involved in different contexts, some of which might counteract the effect. To constantly confirm our causal hypotheses will not bring us closer to a causal understanding. On the contrary, if there was nothing I could do to interfere with the effect, I have learned very little if anything at all about causation. If a lamp lights whenever I press the light switch, irrespectively of what else we do – pulling out the socket, smashing the light bulb, cutting the wire, switching off the power – is this the best proof of causation? Or should I perhaps instead conclude that causation is happening elsewhere, by divine intervention, a prank or magic?