Rationality and reasonableness are often sharply distinguished from one another and are even held to be in conflict. On this construal, rationality consists in means-end calculation of the most efficient means to one's ends (which are usually taken to be self-interested), while reasonableness consists in equitableness whereby one respects the rights of other persons as well as oneself. To deal with this conflict, it is noted that both rationality and reasonableness are based on reason, which is analyzed as the power (...) of attaining truth, and especially necessary truth. It is then shown that, by the rationality involved in reason, the moral principle of reasonableness, the Principle of Generic Consistency (PGC), has a stringently rational justification in that to deny or violate it is to incur self-contradiction. Objections are considered bearing on relevance and motivation. It is concluded that, where reasonableness and egoistic rationality conflict, the former is rationally superior. (shrink)