Sufficient must occur for the necessary to occur. Apples and apple trees really are biconditional statements. Because in real life you need apples to make apple trees (A -> T), and apple trees to make apples (T -> A). You can shorten biconditionals to just A = T What my book says about sufficient and necessary conditions: ——- Event A is said to be a Sufficient condition for event B whenever the occurrence of A is all that is required for the occurrence of B. On the other hand, event A is said to be a necessary condition for event B whenever B cannot occur without the occurrence of A. For example, having the flu is a sufficient condition for feeling miserable, whereas having air to breathe is a necessary condition for survival. Other things besides having the flu might cause a person to feel miserable, but that by itself is sufficient; other things besides having air to breathe are required for survival, but without air survival is impossible. In other words, air is necessary. ——