In an enjoyable post on The Busybody, Loren Rosson asks Did Jesus teach the Golden Rule? with reference to the fourth volume of John Meier's Historical Jesus project. I haven't read the new Meier yet, but I would like to comment on something that emerges from Loren's post. His answer is that Jesus did not teach the Golden Rule -- "Meier shows that the Golden Rule doesn't meet any of the criteria of authenticity, least of all discontinuity"; it is only singularly attested and it is "thoroughly inconsistent with Jesus' demands stated elsewhere".
Are these criteria adequate to the task of establishing that Jesus did not teach the Golden Rule? I don't think so. "Discontinuity", more commonly "dissimilarity" is a notoriously problematic criterion. There must have been substantial continuity between Jesus and his Jewish context, and between Jesus and the first Christians. Käsemann's use of the criterion of dissimilarity only served to create a Lutheran Jesus. Single attestation (Matthew or Q) is, of course, problematic if one is a fan of the criterion of multiple attestation, but those of us who are sceptical about the existence of Q, the independence of Thomas or the independence of John have precious few independent sources anyway. And the alleged inconsistency of this saying with Jesus' other ethical teaching presupposes a use of the criterion of coherence that is at variance with the likelihood that Jesus was inconsistent, like other charismatic leaders of new religious movements (Jack T. Sanders).
But even if these criteria were strong, it is in the nature of criteria in historical research that they cannot demonstrate what Jesus did not say. The point of the criteria, as I see it, should be to help us to work out where the strongest evidence can be found, to adjudicate on what material is the securest in our pool. In other words, we might decide to avoid the use of a particular saying in our reconstruction of the Historical Jesus because that saying is not part of what we think we know for sure. But that is different from saying that Jesus did not say the thing in question.