 The nature of light, its various manifestations and their interconnectedness determine what exists. All existing things can are divided into four classes of existents (from which Suhrawardi’s cosmology is derived). The first category of entities consists of self-subsistent ‘pure’ or ‘immaterial lights’ that are strikingly similar to Leibniz’s monads (Walbridge 2005: 244), those that are associated with the Peripatetic intellects. These lights manifest to themselves are aware of both themselves and what is other than them. These are causally primary, in terms of existence and of efficiency, though they “can differ in intensity and in luminous and dark accidents” (aspects of their individuation). In addition, they are now endowed with the capacity to create and to be “manifest to other immaterial lights”. The second category consists of ‘dark barriers’, bodies whose existence depends on accidental light to make them manifest (as “the effects of lights”) and that are “known only by incorporeal lights”. The third category consists of ‘accidental lights’, inhering in another, such as physical lights, and the “luminous accidents” occurring “in both barriers and in immaterial lights”. The last category consists of ‘dark accidents’, the “qualities of physical things that are not manifest in themselves”, but inhering in another (Walbridge 1992, 49–51, 2004, 213–4; PI, §§107–13, 121). Thus, what distinguishes quiddities is the degree of their luminous intensity within the ontological hierarchies or ‘grades’ (more or less) of light, from the noblest of light, the Light of Lights (God) to the absence of it (matter). 