un- and non-. Both these prefixes are used to produce negative forms of words. W In cases w here they can both be attached to the same root, the resulting un- word is generally stronger and judgemental whereas the non- word is neutral than the non- word. A non-professional tutor is one who is not qualified; unprofessional behaviour contravenes professional ethics. If somebody's methods are described as unscientific, a criticism is usually implied (the methods do not come up to the standards required by science); if they are described as non-scientific, the effect is usually more neutral(the methods come from some other field than science)