Necessary & Sufficient Condition
What is a necessary condition?
It is a condition without which something isn't what it is. (Necessary condition of x = df. condition)
Eagle. Being a bird is a necessary condition of being an eagle. If that condition isn't met, we aren't looking at an eagle.
What is a sufficient condition?
It is a condition the satisfaction of which makes one an "x". (Sufficient condition of x = df. condition)
Eagle. A sufficient condition of being a bird is that you are an eagle. Therefore, all eagles are birds, but all birds are not necessarily eagles.