What does it mean to say that "A causes B?" If you think about it, it's not so simple. When nonscientists talk about causality, they generally mean that the first event preceded the second in time and seemed to be related to its occurrence. Scientists, however, need to be a little clearer. They need to know if exposure to a toxin always makes people sick or only sometimes. They need to understand if a nasty symptom can be caused by one virus or several. It's not enough to simply say that one thing causes another; scientists have to be able to describe the nature of that association. In order to do so, they have developed terminology to describe the causal relationship between two events. They say that causes are
necessary,
sufficient, neither or both.
- If one thing is a necessary cause of another, then that means that the second thing can never happen without the first.
- In contrast, if something is a sufficient cause, then every time it happens the outcome will follow.
When you say that one event causes another you may be saying that the first event is:
- Both necessary and sufficient
- Necessary but not sufficient
- Sufficient but not necessary
- Neither necessary nor sufficient
All four circumstances are types of causality that occur in the real world. Some examples are:
- Necessary But Not Sufficient
A person must be infected with HIV before they can develop AIDS. HIV is therefore a necessary cause of AIDS; however, since every person with HIV does not develop AIDS, it is not sufficient.
- Sufficient But Not Necessary
Decapitation is sufficient to cause death; however, people can die in many other ways.
- Neither Necessary Nor Sufficient
Gonorrhea is neither necessary nor sufficient to cause pelvic inflammatory disease, because you can have gonorrhea without ever developing PID and PID without ever having been infected with gonorrhea.
- Both Necessary And Sufficient
A gene mutation associated with Tay-Sachs is both necessary and sufficient for the development of the disease, since everyone with the mutation will eventually develop Tay-Sachs and no one without the mutation will ever have it.
