But so are sun and earth united by the virtual contact of gravitation. This virtual contact of mover and moved does not go far to explain the union of soul and body. St Thomas happily passes to a further explanation, identifying the union with that of 'form' and 'matter,' that is, of active and determinant with passive and determinable principle. Against which it may be urged that the body has a determinate existence of its own, and powers all its own, mechanical chemical, and many would say, vital also, if we consider the life of cells. This may be admitted or denied, -- it was a theme of endless contention in St Thomas' day, and the strife is not over yet, -- but at least it is to be observed that these various powers are not co-ordinated to the purpose of one human life except by the presence of the soul. Thus the body is the determinable, the soul the determining element, by virtue of which the whole compound becomes one human nature, one man. In this general popular sense, without implication of the details of the Thomist system of matter and form, the General Council of Vienna (A.D. 1312) defined "the rational or intellectual soul to be of itself and essentially the form of the human body."
Of God and His Creatures: 2.56