The Gospel of Thomas Logion 114 is fairly controversial and (like many questions on BH.SE, it seems) has generated a healthy specialist bibliography.1 The translation alone is disputed (or at least discussed!); the one provided by OP is that of Thomas Lambdin (a very fine scholar).
OP: What reasons are there to think that it is or is not an addition?
First, there is no "manuscript" evidence that suggests GThom 114 is an "addition".
Any argument about its status involves debates about the literary development of Thomas as a whole. Stevan Davies is one of those who deems it secondary to some "kernel" of the Thomas tradition.2 His rejection gives a pithy summary of reasons for drawing this conclusion:
- the book's narrative shape, forged in the link between Logion 3 with 113, is spoiled by the odd 114 (which ends the book);
- the location at the end makes supplementing a simple matter;
- it is rhetorically confusing (my wording); and
- it contradicts Logion 22 which refers to the "union of the sexes".
In the wider literature, it is the last point -- the apparent contradiction with Logion 22 -- that carries the most weight.3 (The first three bullet points are fairly impressionistic and do not in themselves carry conviction.) Here's Logion 22 itself (again, Lambdin translation):
Jesus saw infants being suckled. He said to his disciples, "These infants being suckled are like those who enter the kingdom."
They said to him, "Shall we then, as children, enter the kingdom?"
Jesus said to them, "When you make the two one, and when you make the inside like the outside and the outside like the inside, and the above like the below, and when you make the male and the female one and the same, so that the male not be male nor the female female; and when you fashion eyes in the place of an eye, and a hand in place of a hand, and a foot in place of a foot, and a likeness in place of a likeness; then will you enter the kingdom."
The overlap in subject matter is apparent. But, as Plisch notes (see note 3, below), they have quite different starting points: GThom 22 has to do with "non-gendered" infants, while GThom 114 deals with a "gender-defined woman", and Plisch finds at least some possibility of reconciling the two sayings.
Beyond this, one gets involved in much more complex debates about the literary development of the Gospel of Thomas. Two significant voices in this debate are:
Suffice it to say, then, that Logion 114 is difficult to dismiss unless it is take as part of a wider pattern of literary development such as that championed by DeConick in particular. That there may be room for nuancing its translation is certainly the case, but that's a different question -- and one for the Coptic specialists, an exalted company among whom I am not numbered.
- For a decent bibliography of specialist literature on Logion 114, see R. Uro (ed.), Thomas at the Crossroads: Essays on the Gospel of Thomas (T & T Clark, 1998), p. 95 n. 14. For general resources on the Gospel of Thomas, see the typically helpful Early Christian Writings listing. See also the collection of Stevan Davies.
- S. Davies, The Gospel of Thomas: Annotated and Explained (Darton Longman & Todd, 2003 = Shambhala Publications, 2004), see p. 116.
- See, for example, the discussion by Uwe-Karsten Plisch, The Gospel of Thomas: Original Text with Commentary (Deutsche Bibelgesellschaft, 2008), pp. 246-7. Contradiction "led to the assumption that GThom 114 originally did not belong to the Gospel of Thomas but is a secondary addition."