"A deduces B" = "If A holds, then B holds" = "A is a sufficient condition for B" = "B is a necessary condition for A";

"If A is not established, then B is not established" = (negative proposition) "If B is established, then A is established" = "A is a necessary condition for B" = "B is a sufficient condition for A".

"Sufficiency" means that the establishment of one proposition A is enough to ensure the establishment of another proposition B-if we know that A is established, then we can "fully" think that B is established. Necessary condition means that in order to make a proposition B be established, we must have one (because A is the inference of B, the failure of A will negate B, so A is called the necessary condition of B).

Necessary and sufficient conditions are also necessary and sufficient conditions, which means that if proposition Q can be deduced from proposition P and proposition P can also be deduced from proposition Q, then P is said to be a necessary and sufficient condition of Q, and Q is also a necessary and sufficient condition of P. ..

Where there are things, there must be things; If there is a thing case B, there must be a thing case A, then B is a necessary and sufficient condition for A, and vice versa.