Dear all, I have just posted a paper on the use of classes in category theory. https://arxiv.org/abs/1801.08528 Abstract: We examine the use of classes to formulate several categorical notions. This leads to two proposals: an explicit structure for working with subobjects, and a hierarchy of k-classes. We apply the latter to both ordinary and higher categories. Any comments would be welcome. Paul [For admin and other information see: http://www.mta.ca/~cat-dist/ ]