The strange rule of this priesthood has no parallel in classical antiquity, and cannot be explained from it. To find an explanation we must go farther afield. No one will probably deny that such a custom savours of a barbarous age, and, surviving into imperial times, stands out in striking isolation from the polished Italian society of the day, like a primaeval rock rising from a smooth-shaven lawn. It is the very rudeness and barbarity of the custom which allow us a hope of explaining it. For recent researches into the early history of man have revealed the essential similarity with which, under many superficial differences, the human mind has elaborated its first crude philosophy of life. Accordingly, if we can show that a barbarous custom, like that of the priesthood of Nemi, has existed elsewhere; if we can detect the motives which led to its institution; if we can prove that these motives have operated widely, perhaps universally, in human society, producing in varied circumstances a variety of institutions specifically different but generically alike; if we can show, lastly, that these very motives, with some of their derivative institutions, were actually at work in classical antiquity; then we may fairly infer that at a remoter age the same motives gave birth to the priesthood of Nemi. Such an inference, in default of direct evidence as to how the priesthood did actually arise, can never amount to demonstration. But it will be more or less probable according to the degree of completeness with which it fulfils the conditions I have indicated. The object of this book is, by meeting these conditions, to offer a fairly probable explanation of the priesthood of Nemi.