(_b_) Transcendental unity may be either _essential_ (or _substantial_, “unum _per se_,” “unum _simpliciter_”), or _accidental_ (“unum _per accidens_,” “unum _secundum quid_”). The former characterizes a being which has nothing in it beyond what is essential to it as such, _e.g._ the unity of any substance: and this unity is twofold—(1) _unity of simplicity_ and (2) _unity of composition_—according as the substance is essentially simple (such as the human soul or a pure spirit) or essentially composite (such as man, or any corporeal substance: since every such substance is composed essentially of a formative and an indeterminate principle).(137) Accidental unity is the unity of a being whose constituent factors or contents are not really united in such a way as to form one essence, whether simple or composite. It is threefold: (1) _collective_ unity, or unity _of aggregation_, as of a _heap_ of stones or a _crowd_ of men; (2) _artificial_ unity, as of a house or a picture; and (3) _natural_ or _physical_ unity, as of any existing substance with its connatural accidents, _e.g._ a living organism with its size, shape, qualities, etc., or the human soul with its faculties.(138) (_c_) Transcendental unity may be either individual (singular, numerical, concrete, real) or universal (specific, generic, abstract, logical). The former is that which characterizes being or reality considered as actually existing or as proximately capable of existing: the unity of an _individual_ nature or essence: the unity whereby a being is not merely undivided in itself but incapable of repetition or multiplication of itself. It is only the individual as such that can actually exist: the abstract and universal is incapable of actually existing as such. We shall examine presently what it is that _individuates_ reality, and what it is that renders it capable of existing actually in the form of “things” or of “persons”—the forms in which it actually presents itself in our experience. Abstract or universal unity is the unity which characterizes a reality conceived as an abstract, universal object by the human intellect. The object of a specific or generic concept, “man” or “animal,” for example, is one in this sense, undivided in itself, but capable of indefinite multiplication or repetition in the only mode in which it can actually exist—the individual mode. The universal is _unum aptum inesse pluribus_. Finally, we can conceive any nature or essence without considering it in either of its alternative states—either as individual or as universal. Thus conceived it is characterized by a unity which has been commonly designated as _abstract_, or (by Scotists) as _formal_ unity.