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

Editor’s note: this article was originally published on the blog Old School Script. We have taken over its archives and are slowly republishing pieces that have continuing relevance and value. This article was originally published in March 2015. We continue to appreciate Kris Lyle’s willingness to share his writing with our readers here at

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 a larger discussion of these sorts of challenges to semantics, see, particularly, Violi (2001, 55-64; 85-87; 114-117). Or if you’re interested in an entire book on categorization and the nature of linguistic categories, that talks about the classical NSC model as well as a way forward, see Taylor’s (2004) 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?††††


Taylor, John. 2004. Linguistic Categorization. Oxford: Oxford University Press.

From the publisher:
The third edition of the book widely recognized as providing the most readable and clearly articulated introduction to Cognitive Linguistics is fully revised and updated to include the considerable developments in Cognitive Linguistics since 1987. It covers recent research on polysemy, meaning relatedness and metaphors, as well as expanding the discussion of syntactic categories and the relevance of computer simulations.

Violi, Patrizia. 2001. Meaning and Experience. Translated by Jeremy Carden. Advances in Semiotics. Bloomington, IN: Indiana University Press.

From the publisher:
How do we come to give names to things? Why do some things have names while others do not? What is the nature of the relationship between the reality we perceive and the language we use? How do we establish the meaning of words, and how can we be sure that we are sharing the same meanings with others? These and other related problems have been discussed in linguistics, semiotics, the philosophy of language, and psychology for a long time. In Meaning and Experience, Patrizia Violi reviews the most salient issues and themes in this complex debate, taking an interdisciplinary approach. In addition to citing empirical data from research in linguistics and the philosophy of language, she considers the most recent studies in cognitive psychology and neuropsychology, especially in relation to the problem of categorization and its relevance for the construal of linguistic meaning.