  Aristotle distinguishes things that have sentential unity through a combination of expressions (‘a horse runs’) from those that do not (‘horse’, ‘runs’); the latter are dealt with in the Categories (the title really means  ‘predications’[1]).  They have no truth-value and signify one of the following: substance (ousia), quantity (poson), quality (poion), relation (pros ti), location (pou), time (pote), position (keisthai), possession (echein), doing (poiein) and undergoing (paschein). It is unclear whether Aristotle considers this classification to be one of linguistic expressions that can be predicated of something else; or of kinds of predication; or of highest genera. In Topics 1 Aristotle distinguishes four relationships a predicate may have to the subject: it may give its definition, genus, unique property, or accidental property. These are known as predicables.