Determinism: Every object can determine whether it is an element of a set. Without certainty, there will be no trap. For example, "tall classmates" and "small numbers" cannot form a set. This property is mainly used to judge whether a set can constitute a set.

Heterogeneity: Any two elements in a set are different objects.

Disorder: The elements in a set have no order, for example, {a, b, c}{c, b, a} are the same set.