Necessary-and-sufficient conditions in a nutshell (or, Is Jesus a zombie?)

The standard (and deficient) model of categorization

I recently got into a discussion with some co-workers about whether or not Jesus could be considered a zombie. Immediately, the group decided it was necessary to define the terms; and how else better to do this than to pin-point the necessary-and-sufficient conditions for category membership of one entity being labeled ZOMBIE.

Needless to say, I don’t believe this model of categorization holds up to real-life scrutiny. We think about categories much different from this classical Aristotelian paradigm. But I won’t get into that now. First, let’s just get a basic understanding of the central tenets of the necessary-and-sufficient conditions model of categorization (or NSC model, for short).

5 (wobbly) pillars of the NSC model of categorization

  • Every condition (or feature) is a must-have condition. Once the necessary and sufficient features have been established, they must all be present, anytime and every time that category is invoked. In other words, if a feature is considered necessary for a particular category, there's no such thing as exceptions to the rule: it must be present. This is the case even if one’s gut-instinct suggests a certain feature should be included in the category.
For example: “flight” cannot be considered a necessary condition for the category BIRD since some birds, like penguins and ostriches, don’t fly—despite how hard they try.
  • This type of hardline in-or-out mentality is allowed because the NSC model subscribes to the opinion that there exists a strict separation between linguistic knowledge (or dictionary) and world knowledge (or encyclopedic), and that only the former is relevant for category definition and membership.
For example: “flight” is considered encyclopedic knowledge and not dictionary knowledge, the latter of which is essential for defining the category BIRD. What a convenient dichotomy...
  • All members of a category are created equal. In other words, all members of a category have equal membership status. No member of a category is more a member of a category than another.
For example, a robin is just as much a BIRD as a roadrunner or duck or mockingjay(?).
  • All features of a category are created equal. Just like above, this means every single necessary and sufficient feature of a category is considered equal to the next. No single condition or feature is deemed more important than the next. Taken together, whatever features are considered necessary for a specific category are deemed sufficient to define that category. No more features are necessary or critical to have on the list.
For example: if "oviparous", “wings”, “beak”, and “feathers” are considered the necessary and sufficient conditions for membership of the category BIRD, then every individual feature carries the same weight as the next, and not a single other feature is necessary to define the category—like "flight". The named features are fully sufficient. Thus, for instance, a bat is not a BIRD even though it has many of the features. But then what do you do with a kiwi?—it's got no wings or feathers! 
  • The boundary between one category and another is black and white. There are no gray lines or fuzzy cases of whether something belongs to category A or category B. It’s "yes" or "no". There's no "kinda". The options are binary.
For example: if “beak” is a necessary condition for the category BIRD then determining whether or not a toucan, duck, or Big Bird has a beak should not be a problem. Or as previously mentioned, a penguin, ostrich or kiwi shouldn't be difficult to classify as a BIRD. On the other hand, nor should it be difficult to determine that a bat is not a BIRD but a MAMMAL, just like a platypus and a whale. There's no "kinda-BIRD" or "kinda-MAMMAL". Clear as mud, right?

<<For more info on these matters, cf. Violi 2001: 55-64; 85-87; 114-117. Or if you're interested in an entire book on categorization, that talks about the classical NSC model as well as a way forward, see Taylor 2010, Linguistic Categorization.>>

With that said, now would you like to try and establish the necessary-and-sufficient conditions for the category ZOMBIE to see if the resurrected-Jesus fits this mold?