Hi. According to Griffiths the conmutation relations for the angular momentum and spin operators conmutation relations can be deduced from the rotational invariance, as in Ballentine 3.3. For the angular momentum seems logical that it is so, but how is it that rotational invariance leads to spin relations if quantum spin has nothing to do with rotations (as it is emphatically repeated in most books)? Or is it that rotational invariance has actually a different, more abstract, meaning in quantum mechanics than it has in the classical case, as spin does? Thank you for your time.