It is a curious paradox that we think of the physical sciences as "hard", the social sciences as "soft", and the biological sciences as somewhere in between. This is interpreted to mean that our knowledge of physical systems is more certain than our knowledge of biological systems, and these in turn are more certain than our knowledge of social systems. In terms of bur capacity to sample the relevant universes, however, and the probability that our images of these universes are at least approximately correct, one suspects that a reverse order is more reasonable. We are able to sample earth's social systems with some degree of confidence that we have a reasonable sample of the total universe being investigated. Our knowledge of social systems, therefore, while it is in many ways extremely inaccurate, is not likely to be seriously overturned by new discoveries. Even the folk knowledge in social systems on which ordinary life is based in earning, spending, organizing, marrying, taking part in political activities, fighting and so on, is not very 'dissimilar from the more sophisticated images of the social system derived from the social sciences, even though it is built upon the very imperfect samples of personal experience.In contrast, our image of the astronomical universe, of even of earth's geological history, can easily be subject to revolutionary changes as new data comes in and new theories are worked out. If we define the "security" of our image of various parts of the total system as the probability of their suffering significant changes, then we would reverse the order of hardness and see the social sciences as the most secure, the physical sciences as the least secure, and again the biological sciences as somewhere in between. Our image of the astronomical universe is the least secure of all simply because we observe such a fantastically small sample of it and its record-keeping is trivial as compared with the rich records of the social systems, or even the limited records of biological systems. Records of the astronomical universe, despite the fact that we see distant things as they were long ago, are limited in the extreme.Even in regard to such a close neighbor as the moon, which we have actually visited, theories about its origin and history are extremely different, contradictory, and hard to choose among. Our knowledge of physical evolution is incomplete and highly insecure.The word "paradox"(Para. 1) means "（）".