Pondering on the concept of gradation in existence (equivocation), Avicenna and his followers found out that equivocation has nothing to do with essential entities. That is to say, some concepts such as genus, differentia and species are never subject to gradation (being equivocal) but they are always univocal, and since accidents in relation to their instances (referents) are either genus, species or differentia, they are not equally applied to all their referents. Thus, all essential concepts are equally applicable to their instances (being univocal) and it is only the accidental concepts like white and black (not whiteness or blackness) and hot and cold that undergo gradation. Sheikh Shahab AL-DIN SUHRAWARDI, however, maintains that gradation is applicable not only to accidental concepts but also to essential concepts. According to Sadrian philosophers, gradation falls out the realm of concepts, whether essential or accidental. Gradation, according to them, finds its way into real existence – not into its mental existence but only into its external existence. According to Peripatetics, you cannot distinguish between two things except though one of these three ways: through their total essences, through part of their essences and through things outside their essences. They believe one cannot make distinction between two things but in one of these three ways. SUHRAWARDI however, adds a new way of making distinction between them, which is nothing but through gradation in terms of "strength and weakness", "perfection and imperfection" and "priority and posteriority" of their existence. Unlike the previous three ways, here the point of difference is the same as that of commonality.