Collections in Cyc are natural kinds or classes, as opposed to mathematical sets; their instances have some common attribute(s). Each collection is like a set in so far as it may have elements, subsets, and supersets, and may not have parts or spatial or temporal properties. Sets, however, differ from collections in that a mathematical set may be an arbitrary set of things which have nothing in common. In contrast, the instances of a collection will all have in common some feature(s), some ‘intensional’ qualities. In addition, two collections can be co-extensional (i.e., have all the same instances) without being identical, whereas if two arbitrary sets had the same elements, they would be considered equal. Moreover, the ‘best’ collections to create are the ones which are impossible to define precisely, yet about which there are rules and other things to say. For example, the collection of all white cats is not a good collection to create because it’s easy to define with other Cyc concepts, and there’s not much to say about the collection of white cats; but the collection of white collar workers could be a good collection because it is hard to define exactly, yet there are many things to say about it.