A characteristic represents a property of an entity that can be measured (e.g., height, length, or color). We adopt the standard distinction (e.g., as in M. Bunge, 1979) between things (entities) and their properties (characteristics). A characteristic of an entity is observed through a measurement, which further asserts a value of the characteristic for the entity. A characteristic type (e.g., "height") can be associated with many different entities, whereas an individual characteristic (a particular occurrence of the "height" characteristic) is associated to exactly one entity.