Pinnacle concepts are not amenable to detection by the use of keywords. A unit of natural language discourse (UNLD) “refers” to a pinnacle concept “C” when that UNLD uses linguistic expressions in such a way that “C” is regarded as expressed, used or invoked by an ordinary reader of “L.” A reference can have a “reference level” value that is proportional to: the “strength” with which the pinnacle concept is referenced, the probability that a pinnacle concept is referenced or both strength and probability. Pinnacle concepts can be divided into Quantifiers and non-Quantifiers. A Quantifier can modify the reference level assigned to a non-Quantifier. A concept “C,” that is determined to be referenced by a UNLD “x,” after application of its Quantifiers, is said to be asserted by “x.” Concept-based classification is the identification of whether a pinnacle concept “C” is asserted by a UNLD. Concept-based classification can be used for concept-based search.