The elements in a set have three characteristics: certainty, mutual dissimilarity and disorder. Determining whether any object is an element of a set is the most basic feature of a set. Without certainty, it cannot be a set.

Three characteristics of elements in a set

1, certainty

Determining whether any object is an element of a set is the most basic feature of a set. Without certainty, it cannot be a set. For example, "large number" and "tall classmates" cannot form a set.

2. Interrelation

Any two elements in a set are different, that is, the same element cannot appear in the same set. If the elements of two sets {1, 2, 3, 4} and {3, 4, 5, 6, 7} are combined into a new set, then this new set can only be written as {1, 2, 3, 4, 5, 6, 7}.

3. Chaos

The elements in the set are equal, and there is no order. So to judge whether two sets are the same, we only need to compare whether their elements are the same, and we don't need to examine whether the arrangement order is the same. Such as: {a, b, c}={a, c, b}.