I’ll presume, for purposes of this thread, that 1) Q does, in fact, exist and 2) That Thomas is relatively independent from the canonical gospels, and from Q.
That said, it seems inescapable to conclude that Thomas and Q have a similar source, either written or oral–there must be a ground zero from which these two sources draw.
That common source, it is presumed, must be Jesus O’ Naz himself. Indeed, the Jesus Seminar has based virtually all of their work on that simple premise–Jesus is ground zero.
Believe it or not, it’s a simple question that has never been answered. No argument, to my knowledge, has been tendered (save Mack’s sad attempt) for Jesus being the common source. The converse is fatal to the Jesus Seminar. Why have they not addressed it?
In advance, I’ll rebut the only defence of it that I can fathom–these sayings never appear in another context, and are never attributed to anyone else.
This argument is moot. Apocryphal sayings are even today ascribed to all sorts of people, with never a hint that anyone else may have said them–Einstein and Mark Twain probably have dozens all by themselves. So does W. C. Fields. George Washington “Cannot tell a lie,” the list goes on.
Traditions–written and oral–will always gravitate toward the best known version. That best known version attributed them to Jesus. That doesn’t mean that Jesus said them.
I’d suggest the following reasons to be suspicious (there are others, and they can be expanded on as the need arises, but this can get us started).
Perhaps the most common criticism of the Jesus Seminar–these sayings aren’t Jewish. They aren’t Jewish sentiments that have been Hellenized, they’re Greek sayings that have been Judaized. Giving it an extra-spiffy name like “Jewish Cynic” is like putting a yarmulke on Diogenes of Sinope. That doesn’t make him Jewish.
If it looks like a duck, and quacks like a duck. . .Looks like a cynic wisdom collection, probably was a cynic wisdom collection. Drop “Jesus said” off of the common sayings, and you could put whatever name you like in front of them, and they’d serve just fine.
It’s a question of context. Jesus was the only Jew in all of Israel who didn’t believe in a vengeful God? Make that the first Jew in all of Israel? And wait, didn’t John Crossan and Funk say that they’d dislike Jesus if he believed in a vengeful God? Isn’t that serendipitous! It simply doesn’t fit.
More immediate context. No “Sayings tradition” survives from Jesus’ more reknowned (at least then) teacher. Nobody gives any indication that John the Baptist was anything but the most Jewish of Jews. Paul knows nothing of any “Jewish Cynic.” Those immediately preceeding and proceeding Jesus give no indication that we should expect anything like the Common Sayings Tradition to arise.
For these reasons, I suggest the Common Sayings are exactly what they appear to be–a Cynic Wisdom collection. Did Jesus have some Cynic like sayings? Probably. They’re quick, witty, and easy to remember. But the majority of the common sayings are apocryphal. It’s a pre-existent Cynic collection ascribed to Jesus, eventually redacted by Jewish hands, not one that sprang from him.
All commonality can show us is that Q and Thomas had a common source. It cannot show us that this common source was Jesus.