CCC 365 The unity of soul and body is so profound that one has to consider the soul to be the “form” of the body:1 i.e., it is because of its spiritual soul that the body made of matter becomes a living, human body; spirit and matter, in man, are not two natures united, but rather their union forms a single nature.
1 Cf. Council of Vienne (1312): DS 902.