Categorization is the process of dividing everything into categories. This process is a core feature of how he human brain works. Without categorization, we humans would not be able to make sense of the world. It would be absurd to try to build a society free from categorization: Human beings depend on categorization, and every culture or society depends on human beings. However, this is only true for categorization itself. Specific categories can always be modified or replaced.
All specific categories are real in the sense that they exist in the internal realities of those individual human beings who use these categories. Most categorizations are also shared among people. They thus exist as linguistic and cultural patterns between people. In this sense, these categories are real in certain social realities. However, categories do not exist in physical reality: Categories are a part of our understanding of the world, not a part of the world itself. The same is true for all and any other concepts: They exist in language, but not in the physical world. Language is something that we human beings construct socially. Something that we build together, not something eternal given to us from above.
One of the major sources of inspiration for the thesis was the book The Greatest Show On Earth, in which Richard Dawkins explains that people often have a hard time understanding evolution because they believe that species of animals exist... and according to biology professor Dawkins, species of animals does in fact NOT exist. This makes sense if we categorize the reality we live in into three layers of reality: Dawkins is talking about nature itself, not our understanding of it - he is talking about physical reality, not internal or social realities. Categories exist in the minds of human beings and in the cultures between human beings, but they do not exist in the physical world outside human beings. Categories are something that we humans do, not something that exist independently of us.
Categories and other concepts are tools. Necessary and highly useful tools, but tools that can easily turn into mental prisons. We must use these tools, but use them with caution.