An examination of the heresy is of practical value. Our subject-matter is not an oriental antique or a curiosity of the intellect, but a present-day problem of vital moment to the Faith. If we are concerned with a half-forgotten heresy, it is because a study of that heresy serves both as a preventive against error and as an introduction to the truth. The doctor studies disease to ascertain the conditions of health; pathological cases are often his surest guide to the normal; just so the study of heresy is the best guide to orthodox Christology. It was in conflict with monophysitism that the church of the fifth century brought to completion her dogmatic utterances about Christ; and the individual thinker to-day can gain the surest grasp of true Christology by examining the monophysite perversion. With this practical purpose in view, we now proceed to an analysis of the heresy. Monophysitism is a body of doctrine. It is a dogmatic system, in which the individual dogmata are controlled by a principle or dominant idea. As all the particular doctrines of monophysitism depend on this principle, and, as it is not properly a theological concept, but one borrowed from philosophy, we may call it "the metaphysical basis of monophysitism." An intelligent grasp of this basic principle is necessary to an appreciation of the whole system. Accordingly, our first concern is to ascertain and exhibit this metaphysical basis. In subsequent chapters we shall analyse in detail the doctrines specifically monophysite and trace the Christological errors back to their source in metaphysic. THE _A PRIORI_ AND _A POSTERIORI_ IN CHRISTOLOGY