The deficit of such a bridge was first encountered in history by the Pre-Socratic philosophers during the process of evolving a classification of all beings (noun). Aristotle, who wrote after the Pre-Socratics, applies the term category (perhaps not originally) to ten highest-level classes. They comprise one category of substance (ousiae) existing independently (man, tree) and nine categories of accidents, which can only exist in something else (time, place). In Aristotle, substances are to be clarified by stating their definition: a note expressing a larger class (the genus) followed by further notes expressing specific differences (differentiae) within the class. The substance so defined was a species. For example, the species, man, may be defined as an animal (genus) that is rational (difference). As the difference is potential within the genus; that is, an animal may or may not be rational, the difference is not identical to, and may be distinct from, the genus.