The problem for rationality is the concept “boundary”. A boundary is in one aspect (as an assumption) an ambiguity and in the orthogonal aspect (as a deduction) a paradoxical contradiction. The concept is thus an intermediary between “assumption” and “deduction”. It means that not all of these three notions of the concept can be consistent at the same time. And, thus, if we accept assumption and deduction, which we have to, we have to deny the concept “boundary”. This is also what we do in ZFC (ie, the fundament of mathematics).
Another contribution to understanding of conceptualization http://menvall.wordpress.com/