evidence: When you say that one event causes another you may be saying that the first event is: Both necessary and sufficientNecessary but not sufficientSufficient but not necessaryNeither necessary nor sufficient Real-World Examples All four circumstances are types of causality that occur in the real world. Some examples are: Necessary but not sufficient: A person must be living with HIV before they can develop AIDS. HIV infection is, therefore, a necessary cause of AIDS. However, since every person with HIV does not develop AIDS, it is not sufficient to cause AIDS. You may need more than just HIV infection for AIDS to occur. Sufficient but not necessary: Decapitation is sufficient to cause death; however, people can die in many other ways. Therefore, decapitation is not necessary to cause death. Neither necessary nor sufficient: Gonorrhea is neither necessary nor sufficient to cause pelvic inflammatory disease (PID). A person can have gonorrhea without ever developing PID. They can also have PID without ever having acquired gonorrhea. Both necessary and sufficient: A gene mutation associated with Tay-Sachs is both necessary and sufficient for the development of the disease. Everyone with the mutation will eventually develop Tay-Sachs.