Towards the end of his discussion in the Treatise of Human Nature– those parts in which cause-and-effect is under discussion – Hume offers two possible definitions of causality. The first is what other philosophers (such as Elizabeth Anscombe) describe as an ‘exceptionless generalisation’: all causes that resemble ‘c’ are followed by effects that resemble ‘e’. The second definition is the more philosophical idea of ‘necessary connection’. This comes about from an inference of the mind created by repeated ‘constant conjunctions’. The following essay will focus more on the second definition.
Hume himself notes that the second definition could be objected to on the basis that it introduces a ‘foreign element’ into the equation, that of the mind. How can we certain of knowledge based on cause-and-effect if the process relies heavily on the mind of individual observer? Hume seems to foreshadow some of the possible criticisms of his theory of causality. This post will briefly consider this notion of Hume’s and whether the making a causal statement about necessary connection does in fact say something about the observer’s own mind. If this statement is true, does it weaken Hume’s argument in any way?
Firstly, we need to define what Hume means by ‘necessary connection’ and how this relates to the subject of cause-and-effect. Hume first mentioned ‘necessary connection’ when he listed three requirements for cause-and-effect: (1) contiguity in time and space, (2) the precedence in time of the cause over the effect and (3) the idea that the two events are necessarily connected.
Hume then explains what he means by the two events being ‘necessarily connected’. When we observe any object, we do not find any ‘property’ that relates to cause-and-effect. By looking at the object on its own, we can deduce nothing that tells us about possible causes and effects. Therefore, the source of the necessary connection must come from somewhere else. Having dismissed the role of reason in this process – reason can do nothing to prove cause-and-effect demonstratively – Hume eventually concludes that the imagination must be responsible for creating the idea of necessary connection.
But how does the imagination come to form the idea? When we experience something for the first time, we have no reason to believe that this is a ‘universal law’. If I see an apple fall from a tree, and have never seen this before, I could conclude it’s just some freak of nature. If we happen to observe the event a few more times, nothing has changed in terms of our knowledge about the world. Each particular instance in itself does not tell us anything new about the original idea. We’ve already formed that idea in our mind.
However, Hume believes that the repeated constant conjunctions do have one effect: the strength of the original impression increases as we are exposed to more instances of the event. Once we have reached a certain number of linkages between the cause and effect – Hume terms this ‘constant conjunction’ – we convince ourselves that we have found a ‘universal law’. We are convinced that the relationship of necessary connection has been established, so we can be confident that cause ‘c’ will be followed by event ‘e’. Event B comes about from Event A, so if Event A never happens, then Event B will not occur either.
By assessing cause-and-effect in this way, we can see that Hume assigns a critical role to the mind. Because the formation of cause-and-effect only comes about from repeated experience, only those minds with the experience can form the notion of causality. For instance, if someone claims to understand exactly what happens when you drop an apple on the Moon, Hume seems to be saying that this person must have been an Apollo astronaut. Cause and effect can only come about from direct experience. So the cause-and-effect relationships that exist in the mind of any one person reflect the life experience of that person.
This introduces a troubling element of subjectivity into Hume’s notion of causality. Common intuition would tell us that there are certain universal laws – such as the notion of gravity on both the Earth and the Moon – so even if the mind has never received the relevant sensory input, it can still form an acceptable conclusion. This would involve ‘reason’ making the conclusion based on the ideas it already knows. In this case, necessary connection based on constant conjunctions would not be needed, so we seem to have found a relationship of causality that does not require the contribution of an individual mind.
Another possible line of argument against Hume is that based on Elizabeth Anscombe. When we experience an event followed by another event, all we can say is that event B was derived from event A, without giving rise to any form of universal law. If necessary connection is not required – since there is no requirement for deriving universal laws – then the mind of one individual has less of a role to play. Anscombe also argues that there is no need for us to experience repeated occurrences of an event. Just one occurrence is sufficient for us to form a belief based on one event being derived from another event.
So we can conclude by saying that according to Hume, causality based on necessary connection requires the mind of the observer to have undergone an adequate number of constant conjunctions in order to be convinced of a cause-and-effect relationship. However, this does not seem to relate the common intuition. We can be reasonably sure of certain pieces of knowledge – such as an apple being dropped on the Moon moving at a slower rate than that of Earth – without ever having experienced this for ourselves.
Hume’s insistence of the mind having direct experience would seem to be an onerous requirement. If true, only those people who have experience the event for themselves – and a number of occurrences of the event for that matter – can have confidence in knowledge based on cause-and-effect. This therefore does represent an objection to Hume’s theory of causality in that causality should be based on more than just the experience of an individual mind. Hume is resorting to a position of ‘idealism’ in wanting to express laws of nature.