This parable asserts the authority of an ancient scribe to alter a text by interpolating new material that was not found in the text he was copying. There are no parallels to this pericope elsewhere. So, Matthew is the sole source for its contents.

Which source hypothesis has a simpler explanation of this data?

Theory		Relationship
A	Augustine	Mark condensed Matthew; Luke drew on both
B	Griesbach	Luke edited Matthew; Mark condensed both
C	Farrer	Matthew expanded Mark; Luke drew on both
D	Two Source	Matthew & Luke independently edited Mark & Q

Any source theory needs to be supported by redaction criticism. For only a hypothesis that is consistent with each gospel's editorial tendencies at other points can be considered probable.

Hypotheses that Matthew wrote first & was a source for the other synoptic gospels (A & B) must provide a plausible explanation of why Mark & Luke deliberately omit this passage. Hypotheses that Mark was the primary source for the other synoptics (C & D) only have to explain why Matthew decided to add it.

A theory that Luke used Matthew as a secondary source (C) must still explain, however, why Luke chose to omit this passage. A theory that Matthew & Luke independently supplement Mark with other material (D) need only explain Matthew's motives for adding this parable that he knew, but apparently Mark & Luke did not. So, the Two Source hypothesis (D) clearly presents the simplest explanation of the synoptic evidence for this passage.

Mark's omission of this pericope might seem to support Augustine's theory of gospel relationships. If Mark edited Matthew, he must have decided to omit whole sections of the Matthean narrative: e.g., two chapters on Jesus' birth & family background (Matt 1-2), three chapters of Jesus teaching in the sermon on the mount (Matt 5-7), & many Matthean parables (Matt 13:24-30 & 33-53), etc. Yet, this parable authorizes a Christian scribe to introduce "new" items, not to omit "old" material. So, Mark may have deliberately dropped it because it did not support his wholesale suppression of key elements of Matthew's portrait of Jesus.

Such reasoning is circular, however, since it presupposes the very point in question: the hypothesis that Mark edited Matthew. The absence of the parable of the trained scribe in Mark is a demonstrable fact that is open to two quite opposite explanations. Either Mark omitted a pericope that Matthew reported, or Matthew introduced a pericope that Mark did not report. Since this Matthean pericope presents Jesus as explicitly authorizing a Christian scribe to introduce fresh material, its very logic favors the conclusion that Matthew added it to Mark (to justify his own expansion of the Markan text) rather than the alternative.

If one could demonstrate that Mark was a conservative scribe opposed in principle to the very idea that Jesus authorized the introduction of novel elements into his preaching of God's kingdom, then one might argue that Mark had a cogent reason for deliberately suppressing the parable of the trained scribe. But the text simply does not support that conclusion. For Mark reports several kingdom sayings with logical elements that are not paralleled in Matthew.

For example, compare Matthew & Mark's versions of Jesus' inaugural message:

Matt 4		Mark 1
12	Now when he heard	14	Now
	that John had been arrested,		after John was arrested,
	he withdrew into Galilee;		Jesus came into Galilee;
			preaching the gospel of God,
17	From that time
	Jesus began to preach,
	saying,	15	and saying,
	"Repent,*		"The time is fulfilled,
	for the kingdom of heaven		and the kingdom of God
	is at hand."		is at hand;
			repent,*
			and believe in the gospel."

If this Markan text is interpreted as a revision of Matthew's, then Mark deliberately added new wording (black type) that had no basis in its alleged source. On the hypothesis of Matthean priority, not only would the scribe who wrote Mark have changed Matthew's "kingdom of heaven" to "kingdom of God," he would have introduced novel themes -- i.e., the time of fulfillment & belief in the gospel -- that were not part of Matthew's report of Jesus' message.   

Compare, also, the following: 

Matt 16		Mark 9
		1	And he said to them,
28	"Truly, I say to you,		"Truly, I say to you,
	there are some standing here		there are some standing here
	who will not taste death		who will not taste death
	before they see		before they see
	the Son of man		that the kingdom of God
	coming		has come
	in his kingdom."		with power."

If Mark edited Matthew, he has clearly altered his alleged source by introducing a new motif (the coming of the kingdom of God) into a Jesus saying that predicted a somewhat different vision (the appearance of the Son of man).

Furthermore, if one endorses the theory of Matthean priority, one must conclude that Mark deliberately replaced  Matthew's harvest parable (the weeds & the wheat) with another non-Matthean parable (the self-growing seed) that makes quite a different point about God's kingdom.

Thus, the theory of Matthean priority does not offer a coherent explanation of the editorial decisions that it presupposes Mark made.  For if Mark edited Matthew, he frequently did precisely what the parable of the trained scribe claims Jesus authorized. So it is not at all clear why Mark would have omitted this parable. if he knew it. 

Griesbach's synoptic hypothesis provides Mark with a cogent reason for omitting the parable of the trained scribe that Augustine' s does not: its absence in Luke.