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Predicable (Lat. praedicabilis, that which may be stated or affirmed, sometimes called quinque voces or five words) is, in scholastic logic, a term applied to a classification of the possible relations in which a predicate may stand to its subject. It is not to be confused with 'praedicamenta', the schoolmen's term for Aristotle's ten Categories.
The list given by the schoolmen and generally adopted by modern logicians is based on the original fivefold classification given by Aristotle (Topics, a iv. 101 b 17-25): definition (horos), genus (genos), differentia (diaphora), property (idion), accident (sumbebekos). The scholastic classification, obtained from Boëthius's Latin version of Porphyry's Isagoge, modified Aristotle's by substituting species (eidos) for definition. Both classifications are of universals, concepts or general terms, proper names of course being excluded. There is, however, a radical difference between the two systems. The standpoint of the Aristotelian classification is the predication of one universal concerning another. The Porphyrian, by introducing species, deals with the predication of universals concerning individuals (for species is necessarily predicated of the individual), and thus created difficulties from which the Aristotelian is free (see below).
The Aristotelian classification is:
- The definition of anything is the statement of its essence (Arist. τὸ τί ἦν εἶναι), i.e., that which makes it what it is: e.g., a triangle is a three-sided rectilinear figure.
- Genus is that part of the essence which is also predicable of other things different from them in kind. A triangle is a rectilinear figure; i.e., in fixing the genus of a thing, we subsume it under a higher universal, of which it is a species.
- Differentia is that part of the essence which distinguishes one species from another. As compared with quadrilaterals, hexagons, and so on, all of which are rectilinear figures, a triangle is differentiated as having three sides.
- A property is an attribute which is common to all the members of a class, but is not part of its essence (i.e., need not be given in its definition). The fact that the interior angles of all triangles are equal to two right angles is not part of the definition, but is universally true.
- An accident is an attribute which may or may not belong to a subject. That the colour of a human's hair should be black, for instance, is an accident, for blackness-of-hair belongs in no way to the essence of humanity, nor do all humans have black hair.
This classification, though it is of high value in the clearing up of our conceptions of the essential contrasted with the accidental, the relation of genus, differentia and definition and so forth, is of more significance in connection with abstract sciences, especially mathematics, than for the physical sciences. It is superior on the whole to the Porphyrian scheme, which has grave defects. As has been said, it classifies universals as predicates of individuals and thus involves the difficulties which gave rise to the controversy between realism and nominalism. How are we to distinguish species from genus? Napoleon was a Frenchman, a man, an animal. In the second place how do we distinguish property and accident? Many so-called accidents are predicable necessarily of any particular persons. This difficulty gave rise to the distinction of separable and inseparable accidents, which is one of considerable difficulty.