22 But both sets of properties and activities are attributed to the One Person of the Word, because 'One and the same [Person] is . . . truly the Son of God and truly the Son of Man' ( Ep . xxviii, 4. PL . liv, 767). Whence 'In his actions either nature with the co-operation of the other performs what is proper to it; thus the Word performs the part of the Word, and the humanity the part of the humanity' ( Ibid ). In these expressions appears the use of what is called the Common Application of Terms ( Communicatio Idiomatum ), which Cyril vindicated against Nestorius. It depends on the firm foundation that both natures subsist by the One Person of the Word begotten before all ages of the Father and born of Mary according to the flesh in the course of time.
Source: Sempiternus Rex Christus (Vatican.va)