Hume gives two definitions of cause:
“We may define a CAUSE to be ‘An object precedent and contiguous to another, and where all the objects resembling the former are plac’d in like relations of precedency and contiguity to those objects that resemble the latter’ ” ; and:
“A CAUSE is an object precedent and contiguous to another, and so united with it, that the idea, of the one determines the mind to form the idea of the other, and the impression of the one to form a more lively idea of the other” .
As Noonan points out; there are a number of immediate difficulties here, including that the two definitions do not appear to be coextensive, and also we may ask whether Hume’s use of the word ‘determination’ in the second definition does not include the concept of causation, thus rendering the definition circular. Hume himself however believes that the two definitions are two ways of looking at the same object, and he is led to this by his theory of mind. We are led by habit formed by repeated observations to assume that there is a necessary connection between events: it is part of Hume’s scepticism to insist that this is an illusion, albeit an unavoidable one.
What is common to both definitions is the tripartite structure and the first two elements thereof. The use of the term ‘precedent’ simply means that Hume does not accept the idea of backward causation; any cause must be prior in time to its effect. Further, he does not allow the idea of simultaneous causation:
“Some pretend that ’tis not absolutely necessary a cause shou’d precede its effect; but that any object or action, in the very first moment of its existence, may exert its productive quality, and give rise to another object or action, perfectly co-temporary with itself.”
Hume argues against simultaneous causation on two grounds, the first being that it is inconsistent with our observations. His more detailed argument is that if an object is fully existent then that entails it is existent with its full causal powers. If it then fails to have an effect, then it must require an additional partial cause or organising principle to bring about the effect; it is therefore not the sole cause of the effect. There is on the other hand a contradiction in the notion of a sole cause that does not exert itself, because it must lack some element of causal power. Thus a sole cause must act instantaneously if it can do so, and it must be able to do so if the notion of a sole cause is rightly understood. And this outcome is contradictory, because it would immediately populate the universe with effects and destroy any idea of succession.
Contiguity simply rules out the idea of action at a distance: “nothing can operate in a time or place, which is ever so little remov’d from those of its existence” . Again, here Hume relies on the observational evidence that whenever we see a cause in action, it acts locally, and he further supposes that if we do not observe this, it is because the cause is invisible to us rather than that it does not exist, as noted by Zuboff:
“Thus Hume will sometimes speak of ‘secret springs’ of necessity in nature, which are responsible in ways we could never understand for the observed regularities of causation” .
This is highly suggestive of Kant’s later division of the world into phenomenal and noumenal aspects and must be part of what he meant when he wrote that Hume had awoken him from his ‘dogmatic slumbers’.
The final elements of the two tripartite definitions may both be understood as variants of the concept of necessary connection. The final element of the first definition may be viewed as holding that all events of type A are prior and contiguous to all events of type B, then we can say that A causes B. The final element of the second definition means similarly that the idea of A leads us to the idea of B and also that the impression of A leads to the idea of B. This is in accordance with Hume’s theory on the association of ideas.
It is important to note that Hume sees the definitions of cause he gives as complete and exhaustive i.e. this is all we can say on the topic. It will be apparent that this is a quite reductionist approach, which is in keeping with his general scepticism. His view on causation is in keeping with his view on the closely related topic of induction. Hume’s statement of the problem of induction holds that in order to justify inductive reasoning, we need to assert a uniformity of nature principle. This would mean that we could use the past to predict the future. However, there is no way to demonstrate the uniformity of nature principle non-inductively, since we can never use our experience to go beyond our experience. Likewise, we can never use our experience of causation to go beyond our experience thereof and thus what we see is all there is.
Hume is an empiricist and subscribes to the maxim ‘nothing in the intellect not first in the senses’. In his terminology, an impression is a perception of the senses and an idea is a later memory or representation of such an impression.
Hume considers whether it may be proven that a cause is always necessary for any event. He examines four arguments for this and dismisses them all.
The first argument can be phrased after the scholastic fashion ‘ex nihilo nihil fit’, or nothing comes from nothing. However, Hume demands a falsification of the proposition that “any thing can ever begin to exist without some productive principle” and holds that this is both impossible and also consistent with the scholastic phrasing, or “it is a mistake to think that this defeats the logical possibility of a thing simply existing without being produced at all” .
The second argument asserts that nothing can be a cause of itself. Again Hume accepts this, but does not agree that if anything lacks a cause, it must cause itself: “An object, that exists absolutely without any cause, certainly is not its own cause” . To assert otherwise begs the question.
The third argument, which Hume ascribes to Locke, is that anything produced without a cause is produced by nothing; and this impossible because nothing cannot have any causal power. However, Hume again points out that this begs the question because it has already assumed that every event must have a cause.
Finally Hume defeats a semantic argument from the proposition that every effect must have a cause. This again is analytically true in virtue of the meaning of ‘effect’, but does not entail that every event has a cause: “this does not prove, that every being must be preceded by a cause; no more than it follows, because every husband must have a wife, that therefore every man must be marry’d” .
Hume is now in a position to conclude: “Since it is not from knowledge or any scientific reasoning, that we derive the opinion of the necessity of a cause to every new production, that opinion must necessarily arise from observation and experience” . The example given is of a person being fully constituted with faculties but lacking experience. If this person were to observe a billiard ball approaching another, he would have no reason to expect that their collision would result in the struck ball moving off. It could just as well disappear in a puff of smoke or combine with the first ball. It is only our repeated observations of the constant conjunctions of events of type ‘first ball hits second ball’ with events of the type ‘second ball moves’ that makes us think that there is something in the properties of the objects themselves that makes this a necessary connection. As Hume later comments, “there […] is nothing in any object, consider’d in itself, which can afford us a reason for drawing a conclusion beyond it”
However, it is not the case that we are really justified in using our experience to make this argument. As Reid points out, “Necessary connection, he concludes, is a mental association that we project onto objects, rather than something that we discover through our experience of them” and “Our experience of constant conjunction leads our imagination to associate events of the conjoined kinds, and to expect one when presented with the other” . Again this is in good harmony with Hume’s general scepticism as to what we can achieve with limited human intellects, and what is perforce inappropriately but unavoidably foisted on us.
D Hume, A Treatise of Human Nature, Eds. L A Selby-Bigge and P H Nidditch, Oxford University Press 1978 (hearafter “THN”)
H W Noonan, Hume on Knowledge, Routledge Philosophy Guidebooks 1999