The period of three centuries which forms our theme is the central period of the Middle Ages. Its interests are manifold; but they almost all centre round the great struggle between Empire and Papacy, which gives to mediaeval history an unity conspicuously lacking in more modern times. The history of the Church during these three hundred years is more political than at any other period. In order to understand the reason for this it will be well at the outset to sketch in brief outline the political theories propounded in the Middle Ages on the relations of Church and State. So only can we avoid the inevitable confusion of mind which must result from the use of terms familiar in modern life. [Sidenote: Unity of world.] Medieval thought, then, drawing its materials from Roman, Germanic and Christian sources, conceived the Universe as _Civitas Dei_, the State of God, embracing both heaven and earth, with God as at once the source, the guide and the ultimate goal. Now this Universe contains numerous parts, one of which is composed of mankind; and the destiny of mankind is identified with that of Christendom. Hence it follows that mankind may be described as the Commonwealth of the Human Race; and unity under one law and one government is essential to the attainment of the divine purpose. [Sidenote: Duality of organisation.] But this very unity of the whole Universe gives a double aspect to the life of mankind, which has to be spent in this world with a view to its continuation in the next. Thus God has appointed two separate Orders, each complete in its own sphere, the one concerned with the arrangement of affairs for this life, the other charged with the preparation of mankind for the life to come. [Sidenote: Relations of Church and State.] But this dualism of allegiance was in direct conflict with the idea of unity. The two separate Orders were distinguished as _Sacerdotium_ and _Regnum_ or _Imperium_; and the need felt by mediaeval thinkers for reconciling these two in the higher unity of the _Civitas Dei_ began speculations on the relation between the ecclesiastical and the secular spheres. [Sidenote: Theory of Church party.]