"If an element belongs to set A, it must belong to set B" is a definition of "set A is contained in set B". An empty set is contained in all sets (for example, (1, 2)), so if X belongs to an empty set, then X belongs to (1, 2).

In addition, "X belongs to a set" is just a proposition, not necessarily a true proposition. For example, when making some proofs, the reduction to absurdity will be used. A wrong proposition must be assumed to be correct before it can be deduced that it is wrong. Therefore, although the wrong proposition is derived from the wrong proposition, it is also a correct logical process. The element x belonging to an empty set does not exist.

If x belongs to an empty set, then x is not an empty set. Note that "attribution" is the concept of the relationship between elements and sets, and "inclusion" is the concept of the relationship between sets. If set A contains an empty set, then A can be any set; If an empty set belongs to set A, then A can't be an empty set, because there is at least one "empty set" element in A, and then A may be a set, for example, there are no more than two elements in A = {b (set) | set B}.

Or consider the negative proposition of this proposition? We know that the negative proposition of the true proposition must be true; The negative proposition is: if X does not belong to a set, then X does not belong to an empty set. This proposition must hold, because x does not belong to an empty set.

If the topic is changed to p: x 2+x+1< 0, Q: X ∈ (1, 2), then p is a necessary and sufficient condition for q.