Mackie’s INUS condition is a helpful way of thinking about everyday ascriptions of causation. To call a particular event the cause of another event is to say that it was necessary but also part of a set-up that was sufficient.
Trouble begins, however, when we realise that conditions are almost never sufficient for what they cause. If there is even the smallest gap in time between the condition identified and the effect, then in principle, something could have intervened between them. But more generally, we often ascribe causation where logical sufficiency seems too strong. That is surely the case for ascriptions of social causal factors and mental illness. Low economic status merely raises the probability, for example.
There is a different kind of trouble when we ask what underpins the claims of necessity and sufficiency in the INUS model. Mackie suggests that they are truncated arguments from natural laws. And thus what seems to be an account based on Hume’s second definition is really based on the first (and third).
The second general question in the session was: what is the right kind of generality to underpin cause and effect. Note that we cannot say here: connections between general properties where one causes the other (f causes a for given m). We call the right kind of generalities ‘laws of nature’ (by contrast with mere accidents) and we note that they sustain counter-factuals but why?
The most plausible answer to this imposes a kind of systematicity on the laws themselves. So we began with individual causes and rose up to the level of laws. Now we rise to the level of systems of laws which balance universality and systematicity. That puts quite considerable constraint on the ascription of causal factors in the social aetiology of mental illness. We should look for structures of relations between the right kind of variables. Pick the wrong way of taxonomising social factors and we risk arriving at merely accidentally true generalisations. But the challenge is that there is an indefinite number of different ways of categorising social factors.