Alfred Edersheim, The Life and Times of Jesus the Messiah, p.402
Another characteristic of the Parables, in the stricter sense, is that in them the whole picture or narrative is used in illustration of some heavenly teaching, and not merely one feature or phase of it, as in some of the parabolic illustrations and proverbs of the Synoptists, or the parabolic narratives of the Fourth Gospel. Thus, in the parabolic illustrations about the new piece of cloth on the old garment (Luke 5:36), about the blind leading the blind (Luke 6:39), about the forth-putting of leaves on the fig-tree (Matt. 24:32); or in the parabolic proverbs, ‘Physician, heal thyself’ (Luke 4:23); or in such parabolic narratives of St. John, as about the Good Shepherd (John 10), or the Vine (John 15)—in each case, only one part is selected as parabolic. On the other hand, even in the shortest Parables, such as those of the seed growing secretly (Mark 4:26-29), the leaven in the meal (Matt. 13:33), and the pearl of great price (vv. 45,46), the picture is complete, and has not only only in one feature, but in its whole bearing, a counterpart in spiritual realities. But, as shown in the Parable of the seed growing secretly (Mark 4:26-29), it is not necessary that the Parable should always contain some narrative, provided that not only one feature, but the whole thing related, have its spiritual application.
In view of what has been explained, the arrangement of the Parables into symbolical and typical [by Goebel per footnote 16] can only apply to their form, not their substance. In the first of these classes a scene from nature or from life serves as basis for exhibiting the corresponding spiritual reality. In the latter, what is related serves as type (Gr:túpos ), not in the ordinary sense of that term, but in that not unfrequent in Scripture: as examples—whether for imitation (Phil. 3:17; 1 Tim. 4:12), or in warning (1 Cor. 10:6,11). In the typical Parables the illustration lies, so to speak, on the outside; in the symbolical, within the narrative or scene. The former are to be applied; the latter must be explained.
This, it seems to me, is far more in harmony with the fact recorded in the Gospels, that the parables that Jesus explained (the Sower: Matthew 13:1-9 explained in vv.18-23; and the Weeds: Matthew 13:24-30 explained in vv. 36-43) every item in them is explained.