Set
Must be well-defined, so that there is, in principle, a criterion that can be used to decide the membership of any item in the set.
The set of all letters of the English alphabet.
The set of all positive integers.
NOT: The set of all sets that are not members of themselves. (SOASTANMOT). Is SOASTANMOT in SOASTANMOT? If so, then SOASTANMOT violates its own definition; if not,then it SHOULD be in SOASTANMOT. This is known as Russell’s paradox.