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.