“ An Accident is an attribute which may or may not belong to a subject. The colour of the human hair is an accident, for it belongs in no way to the essence of humanity.
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 connexion with abstract sciences, especially mathematics, than for the physical sciences. ”

“ 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) . ”