Causation

In An Enquiry Concerning Human Understanding, David Hume distinguishes two different kinds of experience, namely impressions and ideas. Impressions are what one actually senses when one is having a sensation, whereas ideas are the mental copies or memories of the impressions. Furthermore, ideas are described as less vivid than their corresponding impressions. Hume proposes that all ideas are copies of impressions, and so any idea which confers knowledge about the world cannot be generated a priori.

Ideas can also be combinations of other ideas and impressions. This accounts for how one can have an idea of something that one has not directly encountered. For example, I can have the idea of a pink unicycle, although I have never encountered one. However, I have encountered a unicycle and the colour pink, and so I can form an idea that combines these two impressions. Therefore, although one can form ideas which appear to have no corresponding encounter, they can actually be traced back to their component impressions.

When one observes an event, one experiences the cause followed by the effect, but no necessary connexion between the two. For example, when one switches on a light, one experiences the flick of the light switch followed by the illumination of the bulb, but no necessary connexion prescribing that one must follow the other. One may claim that it is the flow of current that links the two, but this only breaks up the event into a more detailed chain of smaller causes and effects:

The flick of the light switch
Causes
The illumination of the bulb.

This is broken up into:

The flick of the switch
Causes
The completion of the electric circuit
Causes
The flow of current
Causes
The heating of the filament
Causes
The illumination of the bulb.

No matter how much one breaks up an event, one can only reveal more detailed chains of causes and effects, but cannot elucidate a necessary connexion between them. Therefore, it appears that upon observing an event, one only has impressions of the cause and the effect, but not of any necessary connexion between them. Furthermore, since all ideas are copies of impressions, it appears that one can have the ideas of the cause and the effect, but not of a necessary connexion that prescribes that they must be causally linked.

Hume notes that this contradicts our belief about causation. Without the idea of a necessary connexion, causation becomes no more than constant conjunction. In other words, it is no more than events coincedentally following other events:

C occurs, then E occurs.

However, this is not what we believe causation to be. Causation is not just about one event coincidentally being followed by another. It is about the occurrence of the first event making the occurrence of the second event inevitable. Therefore, our belief about causation involves the idea of a necessary connexion between cause and effect:

If C occurs, E must occur.

So, what impression does the idea of necessary connexion come from? As noted above, when one observes an event, one observes impressions of the cause and effect, but not of a necessary connexion. Therefore, there is no external impression that gives rise to the idea of a necessary connexion. Rather, as Hume proposes, the impression is internal. When one observes a cause, one experiences a feeling of anticipation for the occurence of the effect. Hume proposes that this feeling of anticipation is the impression upon which the idea of necessary connexion is based.

The definition of a cause, therefore, is as follows:

C is followed by E, and C always conveys the thought to E.

According to Hume, causation is not an external process or property of the objective world, but has its sources in the subjective experiences of observers. It follows that propositions about causation are not fundamental truths about the world, but statements about our experiences.

References

• Hume D (1748). An Enquiry Concerning Human Understanding.