Is there any way to figure this out? This is the question that I faced as I looked at all the evidence I gathered in my commentary. I had the thought to lay out the Semitisms and compare them with the accretions and the Kernel sayings. So I followed through, not really expecting anything. What I found surprised me. With the exception of two, the sayings in which scholars had identified Aramaisms as possible were all located in the Kernel sayings, including those sayings that may point to a pre-synoptic Aramaic substratum.
Now some people might find this to be coincidence, but I found it compelling, especially when paired with the fact that the content of the Kernel sayings points to a Jerusalem origin. So I concluded based on the big picture that a plausible scenario was that the Kernel was from Jerusalem, written in Aramaic. It was brought to Edessa, Syria, where it moved into Syriac as the Syriac-speaking Christians used it, reperformed it, and added to it. I don't doubt for a minute that in this compositional process sayings in the Gospel of Thomas took on some of the form and vocabulary of Syriac versions of those sayings from other circulating gospel literature. In other words, in the "real" environment of antiquity where orality and memory dominant, what might have begun as an independent version of a saying may not end up that way sixty years later. In fact, we must expect the sayings to take on the character of other circulating materials.
This is the argument that I set forth in both my books.
Unfortunately, this argument seems to have been lost in Nicholas Perrin's recent paper (it is in the poorly edited volume on Thomas that I mentioned in my previous post which also includes my mysticism paper) in which Perrin criticizes me for suggesting a possible Aramaic substratum when Syriac can explain some of these sayings as well and the linguistic evidence is inconclusive. Since he is trying to defend an argument for Thomas being a Diatessaron-dependent gospel, he concludes that the evidence although inconclusive linguistically (the Aramaisms could still be possible he says) points to Syriac.
But I never made the argument that Thomas has an Aramaic substratum because there are possible Aramaisms in Thomas. Professor Quispel and Guillamont were criticized for this back in the 60s. My analysis included much more than linguistic evidence, trying to get us out of this deadlock by looking at the document from a different perspective. For some reason it seems that scholars who try to get out of the box are constantly being shoved back into the box and all the old arguments that they are trying to transcend. This is frustrating to say the least.
My argument was and remains that the vast majority of possible Aramaisms lie in the Kernel sayings, and this suggests to me that it is quite likely from Palestine. This argument is part of a bigger analysis of the Kernel whose content in terms of eschatology and christology also points us to an early form of Jerusalem Christianity.
There are many reasons why Diatessaron-dependence has not been convincing. I am not going to rehearse them all here. The biggest hurdle is our physical manuscript evidence. I am not going to even begin to sort out here the problems of reconstructing the Diatessaron. It is worse than Q. Scholars can't even agree if the original language was Syriac or Greek.
But I can speak briefly to the Greek manuscript evidence for the Gospel of Thomas. P. Oxy. 1 has been determined on paleographic analysis to a date no later than 200 CE. Let's say that this was the autograph, the original manuscript written of Thomas, then that means that it is written as almost as a contemporary to the composition of the Diatessaron (150-170 CE). But remember this copy is in Greek and it is not the original. Perrin says the gospel was composed in Syriac. So this pushes Thomas' composition back even a bit earlier, unless one were to argue that a translation into Greek was almost immediately done in Egypt which is highly unlikely since it takes some time for gospels to become celebrities enough to merit copy and translation and distribution.
Furthermore, P. Oxy. 1's composition was dated by Grenfell and Hunt to no later than 140 CE because the internal evidence for such a dating is compelling. I know that Perrin does not like this argument having said in his first book that most scholars haven't bothered to probe this issue. This is a false statement. Grenfell and Hunt's dating has been generally accepted by scholars because it is compelling based on comparison with other early Christian literature which puts the form and content of this text in the early second century.