Among the parables considered by Bernard Brandon Scott is one from the non-canonical Gospel of Thomas, the parable of the empty jar. While certainly a fascinating entrant in parabolic literature, I am not sure that including this parable serves anything other than the narrow purposes of Scott's larger project of separating the parables from their literary contexts. By doing so, including a parable that we have from only one source - non-canonical at that! - he creates multiple problems. First, ripping this parable out of the Gnostic context of the Thomas' Gospel does structural violence to the parable itself. Since we have no other source for this parable in the canonical literature - or other non-canonical literature - pretending that we can understand it outside the Gnosticism of Thomas leaves me wondering how we do that, exactly. If the intent of studying the parables, apart from their literary settings, involves a certain amount of serious intellectual legwork, including comparing and contrasting across various literary contexts, then how can we arrive at any conclusions outside the context of the Gnosticism of the Thomas Gospel?
While the non-canonical texts, including non-canonical Gospels, are important intellectual and literary sources for seeking a greater understanding of the texts we consider canonical, it is always important to keep in mind that they were not included in the canon for a reason, or multiple reasons; examination of the texts themselves usually reveal these reasons, while occasionally being odd to our modern sensibilities, do make a considerable amount of sense. Taking a single parable from Thomas' Gospel, without any canonical referent, leaves me scratching my head.
Furthermore, Scott's argument as to the antiquity of the Gospel's compilation just doesn't hold up for me. The earliest extant manuscripts we have of the Gospel of Thomas do not predate the beginning of the third century of the common era. They are in Coptic, and while revealing a certain tendency to struggle to render in to Coptic from some original Greek source, this original source has yet to turn up. To argue that the originals may date as early as the mid-first century, without any evidence other than the extant are translations from some earlier Greek version adds nothing to the antiquity of the originals. Again, it needs to be emphasized, the earliest manuscripts of the canonical Gospels predate the Gospel of Thomas by at least, if not more than, a century. Absent evidence, such as a scroll or fragment, that can be dated with a certain amount of precision, any speculation as to the date of the original is just that - a guess.
None of this is to suggest we cannot learn from the non-canonical texts. We can indeed. All the same, we must do that learning with a different set of assumptions; the Gnostic literature, in particular, presents unique issues for any faithful Christian. Does this mean the parable of the empty jar isn't a true parable of Jesus? Of course not. I am just not convinced, given Scott's arguments and the paucity of evidence, that such arguments matter all that much. Furthermore, one can gain much from a study of the parabolic literature in the Synoptics and St. John's Gospel without reference to non-canonical sources to bolster any arguments we make about how they served various purposes in Jesus' ministry.