7 When you hear the word “Son,” you must not think of a nativity after the flesh; but remember that it is spoken of an incorporeal substance, and a simple and uncompounded nature. For if, as we said above, whether when the understanding generates a word, or the mind sense, or light brings forth brightness from itself, nothing of this sort is sought for, or any manner of weakness and imperfection imagined in this kind of generation, how much purer and more sacred ought to be our conception of the Creator of all these!
But perhaps you say, “The generation of which you speak is an unsubstantial generation. For light does not produce substantial brightness, nor the understanding generate a substantial word, but the Son of God, it is affirmed, was generated substantially.” To this we reply, first, When in other things examples or illustrations are used, the resemblance cannot hold in every particular, but only in some one point for which the illustration is employed. For instance, When it is said in the Gospel, “The kingdom of heaven is like leaven, which a woman hid in three measures of meal,” are we to imagine that the kingdom of heaven is in all respects like leaven, so that like leaven it is palpable and perishable so as to become sour and unfit for use? Obviously the illustration was employed simply for this object— to show how, through the preaching of God's word which seems so small a thing, men's minds could be imbued with the leaven of faith. So likewise, when it is said, “The kingdom of heaven is like a net cast into the sea, which draws in fishes of every kind,” are we to suppose that the substance of the kingdom of heaven is likened in all respects to the nature of twine of which a net is made, and to the knots with which the meshes are tied? No; the sole object of the comparison is to show that, as a net brings fishes to the shore from the depths of the sea, so by the preaching of the kingdom of heaven men's souls are liberated from the depth of the error of this world. From whence it is evident that examples or illustrations do not answer in every particular to the things which they are brought to exemplify or illustrate. Otherwise, if they were the same in all respects, they would no longer be called examples or illustrations, but rather would be the things themselves.
Source: Commentary on the Apostles' Creed (New Advent)