Well, I guess that by "subjective concepts", I mean every concept that doesn't have a formal mathematical definition. So stuff like "simple", "similar", "beautiful", "alive", "dead", "feline", and so on through the entire dictionary.

The only theory-of-subjective-concepts I've come across is the example of bleggs and rubes. Suppose that, among a class of objects, five binary variables are strongly correlated with each other; then it is useful to postulate a latent variable stating which of two types the object is. This latent variable is the "subjective concept" in this case.