Hume’s problem of causation takes a similar form to the problem of induction: Causes are not things we can perceive, just as we could not perceive a principle of induction, so we can’t have knowledge of them from experience—yet since they are a thing in the world, we can’t have a priori knowledge of them either. In very brief outline: For an event to be the cause of another, there has to be a special kind of relation between them, a ‘necessary connexion’ which specifies that the one will always be followed by the other. The relation is unknowable by experience, says Hume, because all we actually perceive is regular succession––certain events continually following certain others (a billiard ball always moving when hit, say). We can’t perceive the relation, so our need to posit it is psychological, and irrational.
Has no progress been made since Hume formulated this problem? Popper pointed out in Objective Knowledge that the problem of causation, while similar in form and even taken by Kant to be the same thing as the problem of induction, is directly refuted by Hume’s ‘negative’ solution to the latter. It is irrational to posit causes only if we assume that knowledge is revealed to us by experience—the ‘bucket theory of knowledge’. In the rest of the aforementioned book, Popper sets out an alternative theory of knowledge that does not require the illogical form of inference that Hume himself rejected. According to this alternative, there is no problem in conjecturing a causal relation, for whatever reason, and seeing if it stands up to criticism (empirical or otherwise).
There is no causality—there is only explanation
So that’s an extremely important respect in which progress has been made. Meanwhile, contemporary philosophers have come up with various theories of the nature of causality. Broadly these can be either mechanistic or difference-making, and either monistic or pluralistic. All of them—including, I suspect, the pluralistic accounts—commit the same basic error: namely, in thinking that there is even a problem to be solved here. They assume that there is a thing in the world called causality or a ‘law of causality’ that universally relates events—or in the case of pluralistic accounts, they assume that there are a few such ‘laws’ for different types of events. Both assumptions are wrong, because causes are simply parts of our explanations** of certain phenomena. Clearly we have different kinds of explanations in different fields. Causes in economics are not the same thing as causes in physics, just as explanations in economics draw on different knowledge from those in physics. The attempt to find a relation common to both fields, or even to define one relation that applies automatically in one and a different kind of relation that applies in the other, is not an interesting problem to solve, because it divorces causes from their explanatory roles. And sometimes we don’t even need causes. They are inapplicable at the level of microscopic physics, for example. F. Russo and J. Williamson recently put forward an ‘epistemic theory of causality', which says, perhaps in agreement with me, that the distinction between mechanistic and difference-making causality is a false dichotomy. There are different kinds of evidence of causality, they say, and it is improper to try to infer causal laws beyond that evidence: “Causality is just the result of this epistemology.” I hope to criticise this alternative in a later post. For now, there is no law of causality: There is only a concept of causality that functions as an explanatory tool.
*Viz a planned departure from the syllabus to opine in an honest way that is generally more interesting than the rest of the lecture.** By ‘explanation’ I simply mean accounts of what exists, what it does, and how and why.