The answer to both these questions is either "Yes!" or "No!" depending on the "level" at which one analyzes or chooses to understand the phenomenon of causation. In order to discover whether (and how) causes are sufficient and/or necessary for their effects, we will need to understand to some degree just what sort of thing this phenomenon of "causation" really is.
Philosophers approach the study of "causation" on two different levels. There is the "Humeian" or "Systemic" level, and there is the detailed "Physical" level. Most modern philosophical discussions of causation take place at the Systemic level (for example, Hume, Mackie, Lewis). But there are a few philosophers who approach the topic at the Physical level (for example, Salmon, Dowe, Fair).
Causes and effects are those things that are related by the relation of causation (if indeed it is a relation, there being some debate on the matter). Causes and effects are variously called "objects", "events", "states of affairs", "situations", "facts", "facta", "properties", "tropes", "propositions", "satements", or "conditions". For ease of exposition, I will employ the term "event" to mean any or all of these things.
Hume employs the concept of causation in two separate senses. He employs the Physical concept of causation when he refers to the actual material, physical, scientific connection between an event A and an event B that exists quite independent of any conscious mind. But he employs the Systemic concept of causation when he refers to the belief formed by some conscious mind about the necessary association of some event A and some event B.
In his exploration of the psychology of human beliefs, in his A Treatise of Human Nature, Hume explicitly proclaims that he does not deny the existence of Physical causation, but that it is the nature and origins of Systemic causation that is the focus of his attention. On the one hand, Hume talks about "cause" as a relationship between "objects" that we observe in our experience, and connect in our minds. On the other hand, Hume talks about "secret" and "concealed" causes that we are unable to discover. "'[T]is commonly allow'd by philosophers, that what the vulgar call chance is nothing but a secret and conceal'd cause"[Treatise I, III, xii]. Hume does not deny that there are such "hidden" causes that underlie and explain the precedency and contiguity of observations in our experience. But Hume clearly states that his
"intention never was to penetrate into the nature of bodies, or explain the secret causes of their operation. ... For besides that this belongs not to my purpose, I am afraid that such an enterprise is beyond the reach of human understanding, and that we can never pretend to know a body otherwise than by those external properties which discover themselves to the sense."
Hume's purpose in the Treatise was to "explain the nature and causes of our perceptions, or impressions, and ideas"[Treatise, I, II, v]. Hume, like Descartes and Locke before him, despaired of the ability of science to penetrate the "springs and principles, which are hid, by reason of their minuteness or remoteness"[Treatise, I, III, xii].
Systemic or Humeian causation, then, is the belief that human minds form about the nature of the causal relationship between two events - the cause and the effect. At the Systemic level, the concern is not about how the cause causes the effect. The concern is rather that the cause causes the effect. Physical causation, by comparison, is the more detailed investigation of just how the cause causes the effect. Physical causation approaches the discussion of causation from the perspective of the science of physics. (And as it turns out, most articles from this perspective appear in journals focused on the Philosophy of Science.)
In 1965, J.L. Mackie published a seminal article examining the causal relation from the Humeian perspective. In this article he assumed only two generic events - a cause C and an effect E - that our native intuitions about causal relations stipulate are causally related. From this he inquired whether and how the necessary and sufficient conditions might be applied in a manner consistent with our intuitions.
Mackie reasoned that for a cause to be sufficient for its effect, whenever the cause occurs, the effect must occur. The cause can never occur without the effect occurring. If C is a sufficient cause of E, then the presence of C necessarily implies the presence of E. However, another cause Q may alternatively cause E. Thus the presence of E does not imply the presence of C. Mackie concludes that observation of day-to-day examples quickly reveals that this is much too strong a relation. Our natural intuitions are not consistent with the suggestion that C is a sufficient cause of E. An electrical short may have caused the fire. But electrical shorts do not invariably cause fires.
Next he reasoned for a cause to be necessary for its effect, whenever the effect occurs, the cause must have occurred. The effect can never occur without the cause occurring. If C is a necessary cause of E, then the presence of E necessarily implies the presence of C. The presence of C, however, does not imply that E will occur. But again, observation of examples reveals that this relation is also much too strong. Our natural intuitions are not consistent with the suggestion that C is a necessary cause of E. The fire may have been caused by an electrical short. But fires are not invariably caused by electrical shorts.
Mackie concluded that causes always have their effect in the context of a set of background circumstances. Given different circumstances, the cause will not have the effect that it had in these particular circumstances. Effects may have many possible causes under different sets of circumstances. Causes may have many possible effects under different sets of circumstances.
Mackie proposed that a cause is an INUS condition for the effect. And by that he means a cause is "an Insufficient but Necessary part of an Unnecessary but Sufficient condition" for the effect. The sufficient condition is the entire suite of circumstances that existed at the time of the effect. That entire suite of circumstances is also a necessary condition, since it includes all necessary conditions by definition. Only one (possibly small) part of that entire complex set of circumstances is the part that we identify as "the cause".
To say that the short circuit caused the house fire, therefore, is to say that this particular short circuit was an INUS condition for this particular house fire. The short circuit was an insufficient part of the larger suite of circumstances because it could not cause the fire on its own (other conditions such as oxygen, inflammable material, etc. must have been present), but it was a necessary part of that larger suite of circumstances because (under those circumstances) the fire would not have started without the short circuit. It is a necessary part because, without it, the rest of the conditions are not sufficient for the fire. But this complex sufficient condition is not itelf necessary, since some other cluster of conditions, for example, an arsonist with gasoline, can produce the fire.
To quote Mackie -
"A is an INUS condition of a result P if and only if, for some X and for some Y, (AX or Y) is a necessary and sufficient condition of P, but A is not a sufficient condition of P and X is not a sufficient condition of P" [Mackie, 'Clauses and Conditions', American Philosophical QuarterlyVol 2, 1965, pp. 245-64).
There has been some debate over the details of Mackie's INUS analysis of causation, but his analysis has been largely accepted as the definitive analysis of deterministic causation at the Humeian level. Given no other information other than the existence of two "events" that our native intuitions tell us are causally related, we can therefore conclude that -
(i) caues by themselves are not sufficient for their effects. The things that we identify as causes are but a (possibly small) part of a larger suite of conditions that together are sufficient for their effects; and
(ii) causes by themselves are not necessary for their effects. An effect may have many possible causes.
Of course, Mackie's analysis will not do for what is known as "probabilistic causation". This is the sort of causation active in situations where a certain type of cause event is not always followed by the commonly expected effect event. We identify some event as a "cause", because it is
"an [event] 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."
[Hume, Treatise, I, III, xiv]
But that psychological criterion for a "cause" and an "effect" does not necessitate that the cause is always followed by the effect. Only that the two are associated sufficiently frequently as to couple the two in the mind. And in obvious fact, we often employ the notions of cause and effect when the relationship is only vague - as in "smoking causes lung cancer".
To understand probabilistic causation, we will need to draw on someone like D.H.Mellor. In his "The Facts of Causation", he presents a probabilistic understanding of causation that establishes an event as a cause if and only if it raises the chance of the effect event. (Mellor goes into some extensive detail on just how he wants to understand "chance", but for the purposes of this essay we can simply interpret it as "probability", and leave aside the philosophical difficulties inherent in understanding probabilities.) Based on his analysis, Mellor concludes that
(i) a cause is sufficient for its effect if and only if the cause entails the chances of the effect = 1. (Mellor has a particular "possible world" interpretation of "entails" in this statement, but we can ignore his refinements for the purpose of this essay, and take it to mean the standard logical entailment.)
(ii) a cause is necessary for its effect if and only if the absence of the cause entails the chances of the effect = 0.
Now obviously, for some cause C, the chances of its effect E might be something less than 1 (probability varying continuously between 0 and 1). And if that is the case, if the cause only sometimes is followed by its effect, then the cause cannot be sufficient for its effect. And if the absence of the cause is sometimes followed by the "effect" event, then the cause cannot be necessary for its effect.
So much for two different sorts of analyses from the Systemic perspective. Now let's consider an analysis of causation from the Physical perspective. For this we need to turn to the thinking of philosophers like Wesley Salmon and Phil Dowe. Within the conserved quantity conception of causation, causation is intimately coupled with the conservation laws identified by physical science. At this level of analysis, an event (in the generic sense established above) necessarily involves a change in the quantity of some conserved property of some specific entity - for example, a change in the momentum of a billiard ball, or a change in the kinetic energy of a molecule. The definitions of "cause" and "effect" are based on the source and drain of those changes. If, for example, some amount of momentum is transferred from one billiard ball to another, the change that contributes the momentum is the "cause" and the change that absorbs the momentum is the "effect".
An analysis of causation at this detailed Physical level has the distinct advantage of being immune to the difficulties of "over-determination" and "pre-emption" that plague the Systemic level analysis. On the other hand, being an analysis at the detailed level of particle interaction, it is not the appropriate analysis for such examples of causation as smoking causing cancer, or the raising of taxes causing higher unemployment. Any more than it would be appropriate to discuss the dynamics of internet traffic in terms of electron flow through semi-conductors. Although the former is indubitably based on the latter.
None-the-less, since the essay's title question did not specify the level of analysis from which an answer is expected, an answer from the Physical level must also be considered. And at this level, the answers are the opposite of those from the Systemic level.
(i) causes are sufficient for their effects. The thing that we identify as a cause is the contributor of some conserved quantity that has to go somewhere, and that somewhere is, by definition, the effect(s) of that cause; and
(ii) causes are necessary for their effects. The thing (change) that we identify as an effect is the absorber of some conserved quantity that has to come from somewhere, and that somewhere is, by definition, the cause of that effect.
In summary then, at the Systemic (or Humeian) level of analysis, the level that discusses which events are related as cause to effect, causes are neither necessary nor sufficient for their effects. But at the Physical level of analysis, the level that discusses how causes are related to their effects, causes are both necessary and sufficient for their effects. Which level of analysis is most appropriate (and hence which answers are "correct") is a matter of cognitive convenience within the context of the discussion.
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