This next series of posts considers David Hume’s ideas of causality from the Treatise of Human Nature. Hume wants to know how we develop the idea of ‘necessary connection’ between event A and event B. When we say that two events are necessarily connected, which part of our minds is making this claim, and on what basis do we hold that the connection exists. Hume discusses these ideas in Book 1, Part III.
Section 2: Of Probability; and the Idea of Cause and Effect
Hume first brings up the subject of causality. What do we mean when we say that one object is the cause of another object? As part of Hume’s overall theory, this is an important question since all our ideas are based on impressions. If we want to be clear on our ideas, we must have some way of determining how we came about those ideas in the first place. This suggests that we need to establish a relationship of causality.
In terms of discovering cause and effect, we can’t simply look at an object by itself. There is no such thing as a ‘cause and effect’ quality that attaches to an object. If we happened to believe that such a quality did exist, we would soon find other another object that lacked the supposed quality, but still resulted in the same effect. The idea of cause-of-effect must therefore come about from some form of relationship between two objects.
Hume then identifies the relationship that must exist between two objects in order for us to conclude that the relationship is one of cause-and-effect. The first requirement is that the objects must be contiguous in time and space. Hume does not believe that cause and effect works at a distance. Rather, it we do get the impression of cause working at a distance, this is simply the result of numerous smaller cause-and-effects working together in combination.
However, you could argue that in astronomy, there are many examples of cause and effect working across huge distances in space, such as the way that the Sun holds the entire solar system together, even very distant objects. Hume was probably thinking of more everyday occurrences, such as a way that a voice might travel a long distance by a combination of various echo effects.
After contiguity in time and space, Hume identifies the next key requirement for cause and effect. One object must have a priority in time over the other. In other words, the cause must always precede the effect. To prove why this is the case, Hume relies on a reductio ad absurdum to show that taking the contrary view would lead to an absurd situation (the destruction of all time, according to Hume).
Hume’s argument here proceeds along the following lines:
- We assume that any object that exists is not its own cause. Something else caused it to exist.
- Time by itself can’t produce any change
- However, what if the object itself is the cause of its own existence. Both cause and effect would then happen at the same point in time
- But if that’s the case, everything in the universe would happen at the exact same time, which would result in the annihilation of time as a whole. Succession would never happen as all objects exist at the same time.
Hume is probably correct by saying that there must always be a separation of time between cause and effect. From a human point of view, it might seem as if two events occurred at the same time, but when you examine the objects at a microscopic level, you realise that the events weren’t strictly simultaneous, such as when a chemical reaction occurs.
In addition to contiguity and priority in time, the third key requirement for Hume is the idea of necessary connection. Hume will explore this concept in future sections, but for now he sets down the two questions he needs to answer:
- Why is it NECESSARY for every object to have a cause?
- When we draw an inference of necessary connection, what is the nature of this inference? What part of our minds is responsible for making the inference?