Thanks again for your patience Ill continue to discuss this on user talk pages as youve both requested and I apologise for taking up so much space here I did in fact realise but didnt mention that membership in a set is also as you put it an undefined symbol this is what I meant by asking what it is that makes us consider things as members of sets or groups at all I suppose that my questions about definability in general relate more to a discussion of firstorder logic than to set theory Again sorry for the inconvenience and thanks very much for your time I will look into ZF set theory and model theory more closely I greatly appreciate these suggestions Tastyummy forgot to log in