How do you know which items to include in that set of 24 items, and which to exclude? With different criteria it could be a set of 23 items, or of 25, or 1 or n

How do you know which items to include in that set of 24 items, and which to exclude? With different criteria it could be a set of 23 items, or of 25, or 1 or none or infinite. These 24 items are all unique, as are the excluded ones. The inclusion, the concept of set, the grouping, is not real, it is your criterion, mapping, your abstraction of reality.