The central concept in FST is, of course, fuzziness. Fuzzy sets are categories with graded membership, and constraints upon membership in a category are elastic. Instead of an object either belonging to, or not belonging to a category, as is the case in classical set theory, the object may be more or less a member of a category. For example, a witness to a crime may make a formal statement of what they saw, and this statement may be considered more or less accurate, more or less true, relative to the statements of other witnesses. Fuzziness is contrasted with crispness.
Crispness refers to precise membership within a category.
The crispness of categories we require in scientific theory, description, measurement and discourse is actually abnormal when considered cross-culturally, especially as regards religious knowledge. Indeed, that kind of crispness is rare in each of our own everyday lifeworlds. FST is, by the way, a theoretical approach to crispifying fuzziness. This is unfortunate, because ethnologists utilizing cognitive psychological theories are often uncritical of inherent weaknesses in the cognitive psychological approach. One severe weaknesses is that few cognitive psychology models are grounded in the neurosciences.