To classify knowledge means to reduce the complexity
of reality usmg categories, which allow to control
it easily, and to group and arrange objects and elements
of the reality in order to know them in a
straightforward way. A class is a group of objects,
phaenomena or concepts, which presents one common
characteristic, or a common set of characteristics.
To group objects of reality in classes, it is obviously
required to recognize that these objects own a
characteristic in common. Broader is the "intension,"
that is the number of characteristics a concept owns,
narrower is the "extension," that is the number ofindividuals those characteristics can be applied to or,
according to the terminology used by the Port-Royal
Logic (La Logique OH l'art de penser 1662), respectively
"compréhension" and "étendue." Intensional
logic is mainly the ground for realization of classification
systems, based on the acknowledgment of
specific properties and features of objects.
It IS particularly relevant the distinction we can
draw between genera! principles of bibliographic
classification and principles and laws of the logical
classification. The latter allows to create only two
classes, mutually exclusive, and founds the process
of classification on dichotomous division ^e7'gc«;i.î et
differentiam spedficam, that is the division of a genus
into species, as it was explained in his commentary
on Aristotles' categories, and in particular on the
Substance category, by the Neo-Platonist Greek philosopher
Porphyry (234c.-305c.) (Porphirius 1887)
and later graphically represented by the "tree of Porphyry."
Porphyry's work, known through Boetius'
Latin translation, was essential to the development
of medieval Logic.
Logical classification, to be rigorous, must be realised
according to these particular laws:
- The division of a genus must be carried out using
only one characteristic at a time, with the aim of
creating only two classes mutually exclusive, that
is a class of objects that own the characteristic for
subdividing the genus on the one hand, and the
class of objects that do not own it, on the other
hand. Only two classes-A and not A-are created,
mutually exclusive, without content overlapping;
- The division must be exhaustive, that is to complete
all the possibilities of division without leavmg
objects out, which can not be part of one of
the two classes;
- The division must be done through gradual steps
and it must not contam subdivisions that are unrelated
to the characteristic of the class in object.
It is not possible to take into account characteristics
that are not linked to the aim of the reahzed
classification.