 Take “triangle” as a subject of predication and consider the following two sets of attributes: (i) “being a plane figure enclosed by three straight lines” or simply “being a plane figure”, and (ii) “having three angles” or “having the sum of the internal angles equal to two right angles”. All of these attributes are necessarily true of “triangle” but, according to Avicenna, they are necessarily true of their subject in two quite different ways. The first two attributes, “being a plane figure” and “being a plane figure enclosed by three straight lines” are essential attributes of “triangle” (the former is its genus, the latter is its definition); they are constitutive of it in a way that the other attributes are not. Furthermore, “being a figure” is conceptually inseparable from “triangle” and contained in its definition, in the sense that we cannot properly define “triangle”, in its true nature, without appealing to that notion. By contrast “having the sum of the internal angles equal to two right angles” is not at all required to define “triangle”, even if it is necessarily implied by it, and is in fact an attribute that we typically prove to hold of its subject. Essential attributes of the first kind are captured by definitions, and in general have an explanatory role with respect to attributes of the other kind. They explain the fact that attributes of the other kind hold of their subjects. In Aristotle’s theory of science, these two types of attributes are usually designated by the technical term “per se”.