In his paper, “Causes and Conditions”, J.L. Mackie determines to explain the insufficiencies in attributing effects to causes, as well as the conditions of causes. He begins by suggesting that causes are commonly misunderstood events. A general account of a misunderstood cause might proceed in this way: a cause is an event that comes before another event that is said to be the effect; additionally, the cause is something that is necessary to lead to the effect and sufficient to account for the effect. For Mackie, this sort of reasoning is far too simple and credulous when linking cause to effect. A more complex understanding is needed in asserting causal relationships. Hence, Mackie proposes an alternative way to understand causality as INUS conditions – an “Insufficient but Necessary part of a condition which is itself Unnecessary but Sufficient.”
To illustrate an INUS condition in what Mackie refers to as a, singular cause, he gives the example of a house fire. In this thought experiment, fire investigators have determined that a particular short circuit caused the fire. Yet, the short circuit was not necessary to cause the fire. This is because there could be multiple other reasons that could be attributable to the fire such as; a natural gas leak, an unattended candle, or another variation of an electrical malfunction. Similarly, the short circuit cannot be said to be a sufficient condition for the fire. This is because the short circuit was not isolated, but rather required the presence of flammable material as well as a means not to extinguish the initial beginnings of the fire like an individual possessing a fire extinguisher. Here it is important to note that a circumstance such as the flammable material is said to be a positive condition, while the absence of a means to extinguish the fire is said to be a negative condition. In this way, the investigators understand the short circuit to be unnecessary and insufficient to have started the house fire. So, how, Mackie asks, do the investigators know that the short circuit caused the fire? They know because there are other conditions like, the aforementioned flammable material and the absence of a means to extinguish the fire. When these conditions are added to the condition of the short circuit, a “complex condition” results. This complex condition is interpreted as sufficient for the fire to initiate. Although, this same complex condition is not necessary for the fire to initiate because the fire could have started in alternative ways. However, the short circuit as a part of this complex condition is essential, because the flammable material and absence of extinguishing means could not have started the fire by themselves or in combination. In Mackie’s words, “The short circuit which is said to have caused the fire is thus an indispensible part of a complex sufficient (but not necessary) condition of the fire.” So, in summation of Mackie’s singular cause, the house fire’s cause is an INUS condition in that the short circuit has been determined to be an insufficient (because it could not have started the fire by itself), albeit necessary (because it is essential to the fire) part of a condition which is itself unnecessary (because the fire could have started in other ways), yet sufficient (because the condition allows for the fire to be initiated) result.
Mackie goes on to relate a general causal statement to an INUS condition. He begins with the example of an economist claiming that the restriction of credit leads to, which is to say causes, unemployment. Here, Mackie points out that a “causal field” exists in the form of economics. This is to mean that economics, and an economy in which economics can function, are a multiplicity of causal conditions that differentiate. For a general causal statement, “the causal field is then the region that is to be thus divided” which in this case is the economy in which people are either employed or unemployed. The causal field of the general causal statement also allows for the alleged cause to differentiate, as in, the restriction of credit sometimes happens and sometimes does not happen. So, the economist is claiming that the restriction of credit in this economy is an insufficient (because the restriction of credit could not have directly caused unemployment alone) but necessary (because the economist included the restriction of credit in a causal field that allows for unemployment) part of an unnecessary (because unemployment could be the result of something else) but sufficient (because the restriction of credit allows for unemployment to happen) cause. Using the formal characters that Mackie applies throughout the paper, the economist is stating that for some X (positive and negative conditions) and some Y (another set of positive and negative conditions) combined with A (the restriction of credit) as in (AX or Y), is a necessary and sufficient condition for P (unemployment) in an F (causal field), yet neither A, X, or Y is sufficient on their own accord to account for the cause. Also, in a general causal statement such as this, it is important to differentiate between causes and the cause. If the general restriction of credit can be linked to general unemployment, then it is proper to articulate the restriction of credit as something that causes unemployment. Alternatively, in a singular causal statement, an individual’s restriction of credit can be said to be the cause of their unemployment.
Finally, the aforementioned causal field bears reiteration because it is a factor that functions in both general and singular causal statements. In the former, the causal field is the region in which differentiation occurs, and causes with respect to effects, are determined. Mackie’s example is the sentence, “What causes influenza in human beings in general?” Here, the causal field is “human beings in general” and consequently, differentiation is needed in this term. The new differentiated term could be, “In areas where influenza exists, how do some humans contract influenza, while other humans do not contract influenza?” The key is the differentiation of the cause within a broad spectrum. Mackie writes, “In all such cases, the cause is required to differentiate, within a wider region in which the effect sometimes occurs and sometimes does not.” In a singular causal statement, the causal field is a background in which causes are determined. Mackie’s example is the sentence, “What caused this man’s skin cancer?” There are many causal fields in this sentence and they are to be separated to help determine the cause of melanoma. One causal field could be the man’s job in which exposure to radiation was frequent. A new question could be, “How did this particular man develop melanoma when other individuals at the same place of employment did not?” In this sense, the causal field is now all the individuals that worked alongside the man in a place with copious amounts of radiation. Thus, causal fields figure into INUS conditions as complex conditions. For general causal statements, the causal field is a region like economics in which causes and effects are determined. For singular causal statements, the causal field is a background like a house where a fire started where a causal determination can be determined.