(_e_) Further, the notion of being is the _most abstract_ of all notions, poorest in intension as it is widest in extension. We derive it from the data of our experience, and the process by which we reach it is a process of abstraction. We lay aside all the differences whereby things are distinguished from one another; we do not consider these differences; we prescind or abstract from them mentally, and retain for consideration only what is common to all of them. This common element forms the explicit content of our notion of being. It must be noted, however, that we do not _positively exclude_ the differences from the _object_ of our concept; we cannot do this, for the simple reason that the differences too are “being,” inasmuch as they too are modes of being. Our attitude towards them is _negative_; we merely abstain from considering them explicitly, though they remain in our concept implicitly. The separation effected is only mental, subjective, notional, formal, negative; not objective, not real, not positive. Hence the process by which we narrow down the concept of being to the more comprehensive concept of this or that generic or specific mode of being, does not add to the former concept anything really new, or distinct from, or extraneous to it; but rather brings out explicitly something that was implicit in the latter. The composition of being with its modes is, therefore, only _logical_ composition, not real. On the other hand, it would seem that when we abstract a generic mode of being from the specific modes subordinate to the former, we _positively exclude_ the differentiating characteristics of these species; and that, conversely, when we narrow down the genus to a subordinate species we do so by _adding on_ a differentiating mode which was not contained even implicitly in the generic concept. Thus, for example, the differentiating concept “rational” is not contained even implicitly in the generic concept “animal”: it is added on _ab extra_ to the latter(50) in order to reach the specific concept of “rational animal” or “man”; so that in abstracting the generic from the subordinate specific concept we prescind _objectively_ and _really_ from the differentiating concept, by positively excluding this latter. This kind of abstraction is called objective, real, positive; and the composition of such generic and differentiating modes of being is technically known as _metaphysical_ composition. The different modes of being, which the mind can distinguish at different levels of abstraction in any specific concept—such as “rational,” “sentient,” “living,” “corporeal,” in the concept of “man”—are likewise known as “metaphysical grades” of being.