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