It has been questioned whether this latter kind of abstraction is always used in relating generic, specific, and differential modes of being. At first sight it would not appear to be a quite satisfactory account of the process in cases where the generic notion exhibits a mode of being which can be embodied only in one or other of a number of alternative specific modes by means of _differentiae_ not found in any things lying outside the genus itself. The generic notion of “plane rectilinear figure” does not, of course, include explicitly its species “triangle,” “quadrilateral,” “pentagon,” etc.; nor does it include even implicitly any definite one of them. But the concept of each of the differentiating characters, _e.g._ the _differentia_ “three-sidedness,” is unintelligible except as a mode of a “plane rectilinear figure”.(51) This, however, is only accidental, _i.e._ due to the special objects considered;(52) and even here there persists this difference that whereas what differentiates the species of plane rectilinear figures is not explicitly and formally plane-rectilinearity, that which differentiates finite from infinite being, or substantial from accidental being, is itself also formally and explicitly _being_. But there are other cases in which the abstraction is manifestly objective. Thus, for example, the differentiating concept “rational” does not even implicitly include the generic concept “animal,” for the former concept may be found realized in beings other than animals; and the differentiating concept “living” does not even implicitly include the concept “corporeal,” for it may be found realized in incorporeal beings. (_f_) Since the notion of being is so simple that it cannot be analysed into simpler notions which might serve as its _genus_ and _differentia_, it _cannot_ strictly speaking _be defined_. We can only describe it by considering it from various points of view and comparing it with the various modes in which we find it realized. This is what we have been attempting so far. Considering its fundamental relation to existence we might say that “Being is that which exists or is at least capable of existing”: _Ens est id quod existit vel saltem existere potest._ Or, considering its relation to its opposite we might say that “Being is that which is not absolute nothingness”: _Ens est id quod non est nihil absolutum._ Or, considering its relation to our minds, we might say that “Being is whatever is thinkable, whatever can be an object of thought”.