 As we saw in  Section 3.4,  Book of Letters I,92 sketches out a program for finding a chain of causes of existence, where each cause enters into the quiddity of its effect and has a stronger claim to the title of “being” [mawjûd]. A substance is a cause of existence to the accidents proper to it, and enters into their quiddity, but then there will be a hierarchy of causes of existence within the category of substance itself, until we reach some first substance, “whether this is one or more than one”, which has a stronger claim to the title of “being” than all other substances, and a fortiori than accidents.