Factivity is often taken advantage of in politeness, as ‘believe’ can be interpreted politely as ‘know’ given contextual considerations. Korean has the distinction of being a language that has a productive non-factive ‘know’ (found in Altaic languages), -uro al-, which does not embed a factive presupposition complement, that is nonetheless supported by limited evidential justification (‘believe’ is not). In terms of politeness, this is an excellent way to indicate disagreement with a social superior, as it works both the negative and positive face of the superior. Therefore, this research aims to understand how non-factive ‘know’ is interpreted as compared to other expressions in terms of politeness and appropriateness, in different power situations. It is clearly demonstrated that non-factive ‘know’ is considered to be the most polite way to indicate disagreement. Moreover, participants’ politeness levels were measured psychometrically, revealing how their choices were influenced by their individual politeness characteristics. This factivity phenomenon is given a clear theoretical treatment in relation to politeness. In sum, researching the relationship between factivity and politeness opens new research avenues in languages which take advantage of the non-factive ‘know’ phenomenon.