It's from Psychological Types

Although there are doubtless individuals whose type can be recognized at first glance, this is by no means always the case. As a rule, only careful observation and weighing of the evidence permit a sure classification. However simple and clear the fundamental principle of the two opposing attitudes may be, in actual reality they are complicated and hard to make out, because every individual is an exception to the rule. Hence one can never give a description of a type, no matter how complete, that would apply to more than one individual, despite the fact that in some ways it aptly characterizes thousands of others.

He pretty much admits here that no matter how simple and generalized the functions are, a single description of them will only ever fit the one person the descriptions are made for, or by. You can't make a description that will fit thousands or millions of people that supposedly belong to the same type. We're all exceptions to our type.