Exerpt from Edouard Hugon, Cursus Philosophiae Thomisticae, IIIae-IIa, Metaphysica Ontologica I (Paris: Lethielleux, 1935), pp. 434-6. Translated by Dr. Francisco J. Romero Carrasquillo. Draft version, Copyright © Francisco J. Romero Carrasquillo, Ite ad Thomam, 2017.
On Simple and Composite Ens [...] 
I. – The Notion of Simple Ens. Etymologically, the ‘simple’ means that which is ‘without fold’ (sine plica), or without parts. Hence, ‘simple ens’ altogether excludes plurality and distinction of parts within itself. For this reason, ‘simple ens’ is defined as “that which does not in itself consist of many beings” (id quod in se ex pluribus entibus non constat).
But because there are many different kinds of parts, so there are many different kinds of simple things: the physically simple, the mathematically simple, the metaphysically simple, and the logically simple.
The physically simple is that which does not consist of physical parts, or essential parts, such as matter and form, or of integral parts, or of accidental parts. The mathematically simple is what which is indivisible in the genus of quantity because it is the last terminus of quantity, though it be otherwise physically composite: thus, the point is mathematically simple, but physically it is something composed of matter and form. The metaphysically simple is that which excludes real composition of essence and existence, namely, God. Finally, the logically simple is that which excludes composition of genus and difference. There is also a distinction between the negatively simple, the abstractly simple, and the positively simple. The negatively simple is that which lacks parts due to the paucity and imperfection of its own entity, as the mathematical point, or a substance that is conceived as stripped of its accidents. The abstractly simple (praecisive simplex) is that which is abstracted from its parts on account of its indeterminateness, in the way in which ens in general is most simple, since it cannot be resolved into other concepts. The positively simple is that which excludes parts on account of the perfection of its own entity.
The simplicity that belongs to the ens a se, which is necessary and infinite, is not negative, mathematical, or abstract, for these kinds of simplicity involve imperfection. Rather, the simplicity that belongs to it is essentially positive simplicity. Moreover, it is physical, metaphysical, and logical: that is, it excludes the composition of physical parts, integral parts, accidental parts, the composition of essence and existence, and the composition of genus and difference.
Now, logical simplicity does not belong to creatures, even spiritual creatures; for by their genus and difference beings are restricted to a certain species. Nor does metaphysical simplicity belong to them, for their esse differs from their quiddity. Now, they may possess physical simplicity, which excludes essential or integral parts, but not that simplicity which removes all composition of accidents: for in no created ens is the essence an operative faculty, nor is the faculty identical to the operation itself.
II. – Notion of Composite Ens. By opposition to the simple, the composite is that which admits in itself plurality and distinction of parts, or which in itself consists of many beings. The composite, therefore, taken together in all its parts, is the whole itself, and is divided as a whole. Hence, we must make a distinction between (1) real composites, which are subdivided into (a) essential composites, whether metaphysical or physical and (b) integral and accidental composites; (2) logical composites, which are subdivided into definable and potential; and (3) potestative composites.
Now, all species of composites can be appropriately reduced to five: (1) essential composites, composed of matter and form; (2) entitative composites, composed of essence and esse; (3) integral composites, composed of integral parts; (4) accidental wholes, composed of many accidents, or of substance and accidents; (5) numerical composites, composed of many complete substances which join into a unity of order or of collection.
III. – Positive simplicity of itself implies perfection; hence, an absolutely simple ens is a pure act, or ens per essentiam.
Proof of the 1st Part. Positive simplicity of its own concept excludes whatever is opposed to unity and undividedness, and has the function of containing the thing in unity. But to conserve something in unity is to contain it in esse and in perfection, for one and ens are interchangeable. Therefore, positive simplicity of its own concept imply esse and perfection. Therefore, that which excels in simplicity is greater in perfection; thus plants are more perfect than minerals, animals more perfect than plants, man more perfect than animals, and angels more perfect than man.
Proof of the 2nd Part. The ens per essentiam, or pure act, is an unreceived and unreceptive act. But an absolutely simple ens is an unreceived and unreceptive act, for it does not consist of receptive potency and received act. Therefore, an absolutely simple ens is a pure act and an ens per essentiam.
Now, negative or abstractive simplicity do not imply perfection, either because they abstract from perfection or only deny imperfection.
Therefore, simplicity in the abstract, insofar as it prescinds from positive simplicity, is not a simpliciter simple perfection, as St. Thomas and Cajetan explain.
IV. – Composition of its own concept implies imperfection; hence, every composite is a secondary ens, a caused and contingent ens.
Proof of the 1st Part. Whatever is potential involves imperfection. But a composite, under the ratio of composite, is potential: for either one of its parts is in potency with respect to another, or at least all of its parts are in potency with respect to the whole.
Proof of the 2nd Part. Whatever is the result of something else is a secondary ens, for it is posterior to those things of which it is made up. But the composite is the result of its parts. Therefore, it is a secondary ens.
Moreover, every composite consists of diverse things which of themselves and of their own power do not come together to form something that is one. But those things which of themselves do not come together to form something that is one require a cause to unite them. Therefore, every composite requires a cause, and therefore is a contingent ens and an ens ab alio.
 On this point one may consult St. Thomas, ST I.3 and 9, and his commentators on those questions: Cajetan, Báñez, Sylvius, Gonet, Billuart, Buonpensiere, Satolli, Janssens, Pègues, etc.
 See Hugon, Logic.
 All of these compositions are found in our world, Cf. Hugon, Cosmology.
 Cf. St. Thomas, In IV Sent. dist. 11, q. 2, a. 1, ad 1.
 Cf. Cajetan, Comment. in De Ente et Essentia, c. 2, q. 3.