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94 every detail of these systems has also been so determined. Even so small and apparently insignificant a feature as the classing of the sister-in-law with the sister has been found to lead back to a definite social condition arising out of the regulation of marriage and of sexual relations. If sociology is to become a science fit to rank with other sciences, it must, like them, be rigorously deterministic. Social phenomena do not come into being of themselves. The proposition that we class two relatives together in nomenclature because the relationships are similar is, if it stand alone, nothing more than a form of words. It is incumbent on those who believe in the importance of the psychological similarity of social phenomena to show in what the supposed similarity consists and how it has come about—in other words, how it has been determined. It has been my chief object in these lectures to show that, in so far as such similarities exist in the case of relationship, they have been determined by social conditions. Only by attention to this aim throughout the whole field of social phenomena can we hope to rid sociology of the reproach, so often heard, that it is not a science; only thus can we refute those who go still further and claim that it can never be a science.