The modern history of causation began with the eighteenth century British empiricist philosopher, David Hume’s discussions in his Treatise of Human Nature [1739–40] and his Enquiries Concerning Human Understanding and Concerning the Principles of Morals [1748]. These two works contain substantially the same philosophical material. The Treatise was not well received by Hume’s contemporaries. In his own phrase it fell ‘dead-born from the press’. Hume took this to be the result of its presentation rather than the ideas it contained. Thus the Enquiries is a later revision that differs more in style than content, although for our purposes it presents a clearer account of causation than the equivalent sections in the Treatise. To understand Hume’s account of causation it is necessary first to have some understanding of what would nowadays be described as his philosophy of mind.
Ideas
Hume populates the mind with two sorts of entity: ideas and impressions. There is some difficulty in ascribing a precise view to Hume’s notion of ideas. In modern terms ideas, or thoughts, are construed to be either the bearers of mental content or the content itself. Ideas are thus either the vehicles of thought or the thought itself. For now, think of the distinction between a written sentence and the meaning it bears. It is unclear whether Hume thinks of ideas on the model of the sentence or its meaning.
Impressions
Impressions, by contrast with ideas, are directly experiential. It is through our impressions that we have access to the world. Except for the fact that Hume uses this word for both ideas and impressions, it would now be usual to call impressions ‘perceptions’. Hume also calls them feelings and sentiments. Unconvincingly, he suggests that the main difference between impressions and ideas is one of vivacity. The key claim, however, is that ideas are derived from impressions.With a couple of exceptions, all complex ideas can be derived, via simple ideas, from impressions (the exceptions are causality, as we will see, and unexperienced shades of colours lying between previously experienced shades). There are some fundamental difficulties with views, like Hume’s, which populate the mind with free standing and independent mental states. Once mental states or ideas are conceived as existing independently of how the world is, this opens up both a sceptical question about how we can know that our ideas represent the world correctly but also, more fundamentally, how they come to have any representative powers at all. For now, what is relevant is the use to which Hume puts the distinction between ideas and impressions, both conceived as free-standing mental entities.
Hume’s philosophical methodology
Hume suggests that because ideas are fainter than impressions, they can be confused, leading to errors in reasoning. Impressions having more vivacity, by contrast, cannot. This leads to a central methodological principle: for any problematic idea ‘enquire from what impression that supposed idea derived’ (p. 22). It is this that guides the subsequent discussion of causation.
The account of causation
Having grasped the picture of mind that underlies Hume’s methodology and his suggestion for investigating difficult (philosophical) ideas, we can now turn to Hume’s account of causation.
What is the source of the impression of a necessary connection?
Hume’s account takes as its premiss the claim that a key component of the concept of causation is that of the necessary connection between cause and effect. Hume suggests, however, that this idea is puzzling and problematic.What does it amount to to say that one event necessitates another? How can we find this out? Following the methodological principle set out above, Hume suggests that in order to clarify our understanding of this ‘idea’ of necessity, we should look to the corresponding ‘impression’ from which it is derived. This leads him to consider the different possible sources of our idea of necessity in impressions derived from both outer sense (experience of the outside world) and inner sense (experience of mental phenomena). Hume does not conduct an exhaustive empirical survey. Instead he considers the sort of experience that may possibly be available. However, this inquiry does not turn out to be of any help.
To begin with, our experience of the outer world does not provide the right sort of impression:
When we look about us towards external objects and consider the operation of causes, we are never able in a single instance, to discover any power or necessary connection; any quality which binds the effect to the cause, and renders the one an infallible consequence of the other. We only find, that one does actually, in fact, follow the other. (p. 63)
As Hume remarks, events in the world appear to be independent of one another. We do not observe in them—at least when considered individually—any further power from which we could derive subsequent effects. ‘Solidity, extension, motion; these qualities are complete in themselves, and never point out any other event which may result from them.’ (p. 63)
Nor, according to Hume, is any experience provided by our inner sense of help. Hume rejects the idea that a necessary connection or causal power is experienced when we move our limbs or in the action of the will using six different arguments. But the key idea is that no inner experience of the action of the will can provide an impression of a necessary connection. Mental events, like the physical events experienced in outer sense, appear to be independent of one another and of actions. They are conjoined but not connected. It is quite consistent with any mental event that any other event succeed it.
This result appears to put Hume’s general claim that ideas are derived from impressions under threat. If there is no impression from which the idea of a necessary connection is derived, how could one even undertake the search that Hume describes. Hume’s own response does not immediately seem to address this point. Instead he suggests that our idea of causality derives not from any single case but from the general connection between events of one sort and events of another sort.
If a general relation is observed then: we make no longer any scruple of foretelling one upon the appearance of the other . . . We then call the one object, Cause; and the other, Effect . . . It appears, then, that this idea of a necessary connection among events arises from a number of similar instances which occur of the constant conjunction of these events. (p. 75)
In effect Hume’s method works in a negative way. When we go from the (less vital) idea of causal necessity to the (more vital) impressions from which the idea is derived, it becomes clear that all there is, is constant conjunction. Whatever our initial assumption, there is no further sense of causal necessity.