I just wanted to add two points to my previous posting.
The fact that Iamblichus was able to reproduce sections of the
Protepticus of Aristotle in his own Protepticus shows that at least
that exoteric work of Aristotle was still extant, or at least parts
of it were, in the late third century. While this isn't proof for On
the Good, it is at least worth mentioning in this context.
Also I wanted to mention the quotation of Speusippus made by Proclus
in Book 7 of the Parmenides Commentary (in Moerbeke's translation).
While neither proclus nor Speusippus refers to the Unwritten
Doctrines in it, this passage does discuss the One (as being
unaspected and unaspectable). It's easy to associate this bit with
Plato's Parmenides and of course that is the context in which Proclus
is reproducing it. But this also shows one problem with trying
to 'disentangle' potential references to the U.D. in later writers;
quotations made without attribution to author or to a specific work,
which discuss many of the matters concerned in interpreting the U.D.
could also as easily be concerned with issues of the first and second
hypotheses of the Parmenides. Add to this any of the Neopythagorean
(or even earlier Pythagorean for that matter) discussions of the One
and Infinite Dyad, etc, and how they too are sometimes alluded to
rather vaguely in a later writer, and there is potentially a three-
Or so it seems to me, and I have spent a goodly amount of time trying
to disentangle these for myself at least. (I am certainly aware there
is not universal acceptance of the U.D., but as you can tell I give
them credence, and I really don't see how the objections at this
point are valid.) I have also come to the conclusion that this may
just all be, aside from the point of trying to determine a
developmental chronology, rather a distinction without a real
difference, at least from the point of view of anyone writing in the
third or later centuries.