Language is for communication: language is public. The correctness of a meaning comes from a language community, and only from there (dictionaries are a staging post; either they reflect the language communities usages or they are wrong). The proximate assignment of meanings to words within brains is likewise either in line with usage or wrong. You need a brain to understand a word, but a brain cannot grant correctness to any arbitrary meaning-word assignment,

So you ought not do as you damn well please. Especially if you enjoy serial killing.

The "meaning theory -- the idea that disagreements about morality are disagreements about the meanings of "should", "ought" and "good" -- is put forward to explain a fact about disaggreements. The "theory theory" is an alternative explanation. It is not the case that a disagreement about how to apply a word must be a disagreement about its meaning. In fact, disagreement about a implies common ground -- otherwise it is a case of two people talking past each other. It is not the case that understanding the dictionary meaning of a word, at the level of ordinary linguistic competence, gives competence in applying it. I understand the meaning of the word "cantonese", but I could not distinguish it from mandarin. In many areas, it requires specalised knowledge to apply a word. So, on the theory-theory, there must be a meaning of good/should that anyone can produce. Since there is no disagreement about the basic meaning, we would expect it sound obvious, a truism. I propose that the truisms in question are something like: good acts are praiseworthy, bad acts blameworthy. People who have a theory of X can make assignements, and can explain their rationale for doing so. People who do not have a theory of morality, most people, may or may not be able to make asignments intuiitively, but will not be able to explain their rational. Theists (and philosophers) can answer moral questions because they have a theory, not because they are aware of a some meaning that is denied to other English speakers.

And what Parfitt is offering, and what many people can't, is a theory of "ought", not a definition.