_Idem scire_ is a good thing; but men of all nations may know the same thing, and yet remain strangers one to another. _Idem velle idem nolle in re publica, ea demum firma amicitia est_. The nations will at last attain firm friendship one with another in the day when a common moral will controls the scope of public things. And when they have attained this friendship, then on a far higher level of economic development and with an improvement by each nation of its talent which is almost entirely new--they will have found again, if in a different medium, something of the unity of mediaeval civilization. BOOKS FOR REFERENCE W.J. Ashley, _An Introduction to English Economic History_, vol. i, pt. 2, ch. 3; vol. i, pt. 2, ch. 6. Longmans. Lord Bryce, _The Holy Roman Empire_. Macmillan. A.J. and R.W. Carlyle, _Mediaeval Political Theory in the West_. W. Blackwood. H.W.C. Davis, _Mediaeval Europe_ (Home University Library). Williams & Norgate. _Encyclopaedia Britannica_ (11th edition), articles on 'Crusades' and 'Empire'. J.N. Figgis, _Churches in the Modern State_, Appendix I. Longmans. Bede Jarrett; _Socialist Theories in the Middle Ages_. T.C. and E.C. Jack. E. Jenks, _Law and Politics in the Middle Ages_. Murray. F.W. Maitland, _Political Theories of the Middle Ages_, translated from Gierke's _Das Deutsche Genossenschaftsrecht_. Maitland. Cambridge University Press. R.L. Poole, _Illustrations of Mediaeval Thought_. Williams & Norgate. H. Rashdall, _Universities of Europe in the Middle Ages_. Clarendon Press. A.L. Smith, _Church and State in the Middle Ages_. Clarendon Press. H.O. Taylor, _The Mediaeval Mind_. Macmillan. E. Troeltsch, _Die Soziallehren der christlichen Kirchen_ (II. Kapitel). P. Vinogradoff, _Roman Law in Mediaeval Europe_. Harper. FOOTNOTES: [Footnote 15: I should like to dedicate this essay to my friend and old pupil, the Rev. Bede Jarrett, O.P., to whom I owe much, and to whose book on _Mediaeval Socialism_ I should like to refer my readers.] [Footnote 16: Pirenne, _Revue Historique_, liii. p. 82.] [Footnote 17: _De Vulgari Eloquio_, 1. viii.] [Footnote 18: _De Monarchia_, 1. x.] [Footnote 19: Cf. Carlyle, _Mediaeval Political Theory in the West_, ii. 219-22.] [Footnote 20: Cf. E.R. Bevan, _Stoics and Sceptics_.] [Footnote 21: _Die Soziallehren der christlichen Kirchen_, p. 242.] V UNITY AND DIVERSITY IN LAW