On his blog, Drummond Reed riffs on some comments I recently made on the OpenID General mailing list concerning the persistence of identity. The upshot is that we should be more aware of the attribute "persistence" when discussing identity systems. This is particularly true when identity systems are intended to model "numerical identity" -- normally assumed to persist -- rather than the often more plastic attributes of qualitative identity.
To better illustrate the issues in this discussion, I propose that we update the well-known "Zooko's Triangle" and create a "Pyramid" that adds the attribute "persistence" to the attributes "Memorable," "Secure," and "Global" which are already included in Zooko's Triangle.
The argument made by Zooko's Triangle is that no naming/identity scheme can provide all three of the attributes Zooko considers essential metrics of identity systems. For instance, while you might be able to build a "Secure and Global" naming system, in doing so, you would undoubtedly need to use identifiers that were not "memorable" -- at least not by mere humans. The importance of these three system attributes and the difficulty of producing systems which provide all three is generally well accepted by those in the naming/identity business.
To the three attributes or axes of Zooko's Triangle, we need to add a fourth axis or dimension which is "Persistence" (i.e. that which relates to the difficult and controversial subject of Identity over Time). The result is a pyramid which allows us to better model constraints on the universe of achievable identity systems. For any of the three traditionally recognized attributes, we need to ask the question "For how long?" (e.g. For how long will an identifier be memorable? For how long will an identity system be secure? What determines the period of time during which a globally unique identifier can be considered "global?")