Talcott Parsons (1902-1979) was usually thought of as quintessentially sociologist's sociologist, and so he was in a sense. Ever since he published“The Structure of Social Action”(1937), he put forward a definition of sociology as a special science and continued to fight against all kinds of synthetic sociology from his stand-point of analytic realism. However, so far as we can conclude from reading his books and articles, it cannot be denied that his scientific concern was directed not so much to sociology itself in his sense than to social system theory which embraced not only sociology, but also economics, politics and religious study, just as in the case of synthetic sociology. Hence he might well be said to have gone far beyond sociology alone. But it is, of course, very questionable whether he could be credited on that account as an excellent sociologist in his sense, because it is too clear that his contributions in this field was far fewer and devoid of qualitative brilliance to be compared to those in the social system theory. In a word his most enduring legacy to us is presumably not his sociology, but his social system theory. And this is the very final and unexpected conclusion to which I reached after having reflected over his lifelong work in its totality.