Topics Logic

Question I am having trouble understanding the difference between a 'necessary' and a 'sufficient' condition (in philosophical usage). Would I be right in thinking that the former is a condition that must be present in order for something to happen, while the latter is merely 'enough', i.e. that no other condition needs to be met (while with a necessary condition others can be met)?

Accepted:
January 24, 2006

Comments

An example might help you understand the difference. Being at least 35 years old is a necessary condition for being President of the United States. You cannot be President unless you are at least 35 years old. But being at least 35 years old is not a sufficient condition for being President, i.e., to be President, it is not enough that you be at least 35 years old. Otherwise, I would be President.

That's right.

A is a necessary condition for B: B obtains only if A obtains.

A is a sufficient condition for B: If A obtains then B obtains.

A can be a necessary but not a sufficient conditiion for B. Example: having legs is a necessary but not a sufficient condition for walking (the legs also need to be used in a certain way).

And A can be a sufficient but not a neceesary conditon for B. Example: dropping a ball is a sufficient but not a necessary condition for moving it (the ball could also be moved by by throwing it upwards).

This is perhaps the most common way of defining these expressions. And I think it reasonably captures our fairly straight-forward and unproblematic usage. But as always, things are more complicated when you scratch beneath the surface. To do so, look at the entry on necessary and sufficient conditions in the Stanford Encyclopedia of Philosophy.