Necessary condition is a form of relation in mathematics. Without a, there must be no b; If there is a without B, A is the necessary condition of B, which is marked as B→A and read as "B is included in A". Mathematically speaking, if condition A can be deduced from result B, we say that A is a necessary condition for B. ..

If there is no material feeling A, there must be no material feeling B. That is to say, if there is material feeling B, there must be material feeling A. Then A is the necessary condition of B. Logically, B can deduce the necessary condition of A being B, which is equivalent to the sufficient condition of B being A. ..

Suppose a is the condition and b is the conclusion:

(1) If B can be deduced from A and A can be deduced from B, then A is the necessary and sufficient condition of B (A=B).

(2) If B can be derived from A and A cannot be derived from B, then A is a necessary and sufficient condition for B (A? B) yes.

(3) If B cannot be derived from A and A can be derived from B, then A is a necessary and sufficient condition for B (B? Answer.

(4) If B cannot be derived from A and A cannot be derived from B, then A is a necessary and sufficient condition for B (A? B and b? Answer.