Unlike Mark & Luke, Matthew concluded Jesus' explanation of his rationale for speaking in parables by commending his disciples for seeing what others did not [Matt 13:16-17]. This commendation underscores the distinction between imperceptive outsiders & the well-informed inner circle of Jesus' followers. By placing this saying at the climax of this pericope, Matthew clearly gave Jesus' explanation a positive focus that is missing in the parallel accounts of both Mark & Luke.

Mark has no equivalent of Matthew's climactic commendation of Jesus' disciples anywhere in his gospel.  Luke, however, includes a very close paraphrase of this saying in Jesus' celebration of the successful mission of  70 disciples that he locates 16 scenes after the conclusion of this discussion on parables:

It is conceivable, even probable, that Jesus used the same saying on more than one occasion.  But the use of a single saying in different narrative contexts by authors whose texts show some type of literary relationship complicates the question of which author was dependent on what source.

The absence of a Markan equivalent to this parallel saying in Matthew & Luke does not prove that Luke used the gospel of Matthew, as the hypotheses of Augustine, Griesbach & Farrer assume. That would be the case only

• if Matthew created this aphorism & it never circulated in oral form; or
  

• if Matthew & Luke recorded the same saying at the same place in the narrative where it is missing in Mark.

But the first condition is beyond proof & the second not true. The fact that Luke uses the same non-Markan saying as Matthew is evidence only that these two authors knew the same material. The question is: which editorial scenario presents a more plausible explanation of the fact that each author records it in a different place in his narrative?

• Luke took this saying from Matthew's discussion of Jesus' use of parables but reserved it for use 16 scenes later, after the mission of the 70 (an incident not included in either Matthew or Mark); or
  

• Luke got this saying from a source other than Matthew & recorded it in his gospel without consideration of how Matthew had used it.

If an editor modifies a passage from a written source, we can assume (s)he was trying to clarify and/or correct it. If that editor omits a passage from the text being revised, we can assume that for some reason (s)he did not find it essential or even useful. But if an editor drops lines from one scene & then reproduces virtually the same lines in another literary context, then we have to assume that (s)he liked the idea but thought it would function better somewhere else than where it was previously recorded.

The block move & drag features in modern word processors have made the transposition of text a relatively simple operation. Without these tools, this electronic text would have taken far longer to compose than it did. 

In the world of handwritten manuscripts or even typewritten texts, however, the  transposition of even a single sentence more than a line or two in any direction is a cumbersome maneuver that consumes precious time & material. The words to be moved have to be marked in the original text. Then they must be copied between the lines or in the margins at the place in the text where they are to be inserted. Many ancient manuscripts, including those of the gospels, provide ample evidence of such editorial practices (Codex Sinaiticus, for example). In antiquity writing materials (papyrus or parchment) were so costly that these emended texts were generally not thrown away.  The next time they were copied by a scribe, however, the words marked for deletion were omitted, while marginalia & interlinear wording was copied directly into the text. 

Moreover, in antiquity an editor would have had to have a rather compelling reason to transpose any saying or other passage from one literary context in an extant work to a position much later in a new revised version.  For scribes did not have electronic clipboards where they could store lines they had excised from one context until they finally got to the point in the revised composition where they thought it would fit better.

In terms of both rhetoric & logic, however, Luke's presentation of this aphorism is actually inferior to Matthew's.

• Both verses of Matthew's version contain rhetorically balanced references to seeing & hearing; while Luke's first verse lacks any reference to hearing.
   

• In Matthew this saying is linked by catchwords to its context, while in Luke it is not. Matthew's balanced reference to eyes & ears echoes the pair of references to eyes & ears in the citation of Isa 6:10 which prefaces it. There are no word links between this saying & the thanksgiving to which it is appended in Luke.
   

• In Matthew this commendation is used to distinguish Jesus' followers from outsiders who have just been described as imperceptive. In Luke the rationale for this commendation being addressed to the disciples is less clear, since the thanksgiving that precedes it celebrates a paradoxical revelation that is known only to infants.
   

• In Matthew's context  "what you see" has a clear antecedent (Matt 13:11: the secrets of the kingdom of heaven), but in Luke's context it does not.

So, if Luke got this commendation from Matthew, he not only plucked it from a rhetorical context in which it fit very well, he made Jesus' reason for uttering it unclear.  Such deliberate obfuscation is hardly credible of Luke, since he insists that he has followed all things exactly (πᾶσιν ἀκριβῶς) & written things in order (καθεξῆς) [Luke 1:3].  It is easy for anyone who compares these two gospels to see that here as elsewhere Luke does not follow Matthew's order. And it should be obvious to careful readers that Matthew has done a better job than Luke in integrating this aphorism with the surrounding material.

The Two Source hypothesis offers a simpler explanation of the location of this non-Markan aphorism at different points in Matthew & Luke.  For if Luke did not use the gospel of Matthew but rather the same source [Q] from which Matthew also got this saying, then one does not have to explain why he dismantled a well-constructed passage in Matthew only to recall one element in a less appropriate context. All it requires is the historical sense to recognize that Luke & Matthew could have both had access to copies of a document [Q] that was later lost -- like the works of Papias, Tatian, Origen & many other early Christian writers -- & cited passages from it without consulting each other's  gospel.