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  "body": "\n## Summary\n\nIn set theory, a class is a collection of sets defined by a property that all its members share.\nEvery [[set|0BSJ7RQ9BW]] is a class, but some classes are too large to be sets; these are the proper classes.\nExamples of proper classes are the class of all sets and the class of all [[ordinals|0YSCSX5YZC]].\nClasses let one speak about such large collections without the contradictions of [[Russell's paradox|0HESMMSYEG]].\n\n## Explanations\n\n### L1 human\n\nA class is the collection of all sets with some property, such as \"being a set\" or \"being a group\".\nSmall collections, such as the even numbers, are sets.\nSome collections are too big to be a set: if the collection of all sets were a set, one could form Russell's paradoxical set from it.\nSo one calls them proper classes, and does not allow them as elements of other collections.\nIn [[Zermelo–Fraenkel set theory|0HE5WDQ9FJ]], a class is just a way of talking about a formula; other theories treat classes as objects.\n",
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