This is a summary of my views of ens commune that I formed from my previous article and the resulting discussion. I suppose that some of this is a positive assertion against some of the claims that ho_mathetes made in the previous post. I would still like to respond directly to his position, but I thought it would be helpful to late out my position as a formal whole as well.

In the pursuit of metaphysics there are many challenges that present themselves to the would be metaphysician. In contemporary thought, such challenges often arise from thinkers who oppose the very idea of metaphysics. Such thinkers as Kant and his present day adherents come quickly to mind. However, aside from these attacks from “outside” of metaphysics, there are other challenges, even for one who approaches the study with a good will. I do not speak merely of the challenges that arise from the difficulty of the matter, but there is a real question about whether metaphysics is even possible as a philosophical pursuit. A sign of the force of this difficulty is that in the eleventh book of the Metaphysics (of fourteen) Aristotle still is seeking whether there is such a science that can treat being qua being.1
Of particular concern to Aristotle is how to treat being qua being as something one. This will be of vital importance to metaphysics as a science. If being qua being cannot be understood as something one then there will not be one science. Nor would it seem possible to treat of first principles in a unified way and thus defend them accordingly. Consequently, it is of great importance to Aristotle that we see how to consider being qua being as something one.
This is where our present inquiry enters. St. Thomas Aquinas refers to this unified treatment of being qua being, as ens commune, or common being. In his preface to his commentary on Aristotle's Metaphysics St. Thomas refers to ens commune, naming it the subject of the science and a genus. It is the task of this paper, therefore, to examine what ens commune is. At the conclusion of this paper we should see that ens commune is only considered a genus in an extended sense and is thus an ens ratione with a foundation in things, unified through the notion of substance.
We will follow Aristotle's example and begin with what is more known to us and then proceed to what is more known by nature.2 It is important, therefore, to give a preliminary consideration to this concept of ens commune. Literally translated it means common being. Ens does not, however, have any connotation of action. Rather is refers to a being, or better yet, an entity. It should therefore be always taken as a noun. Common, here, can be understood in two ways. As St. Thomas argues, a principle is said to be common either through predication, or as a common cause.3 It remains to be seen which sense of common should be taken in ens commune.
There are a number of presuppositions that this paper will impose upon its readers, first of which is a presupposition of Aristotle's work in philosophy of nature. I will therefore be pursuing a particular path into metaphysics, which I will not here defend. My reasons for choosing to enter metaphysics through philosophy of nature are two-fold. First, this has been the traditional mode of proceeding for centuries. Second, I only see how to manifest the concept of ens commune by beginning with the philosophy of nature. I acknowledge that there are varying opinions about the necessity of treating physics before metaphysics, but I will not treat of that matter here.
To begin, it is clear that the subject of the philosophy of nature is mobile ens4 and its principles. It is this sort of ens that we are first and foremost familiar with. This is because we are composite beings and we know through our senses. Mobile ens is material ens, and therefore we know it as sensible ens. What is more, at first ens is equivalent to material ens, since anything more is beyond our experience.
Our idea of ens undergoes a radical change in the course of the philosophy of nature. This occurs when we prove the existence of the immortal human soul5 and of the first mover.6 What is significant about both of these is that they are both immaterial. Ens is now said in an entirely new way, since our studies in the philosophy of nature have compelled us to admit such beings. We consequently must expand our understanding of what it is to be an ens. This is what the concept ens commune attempts to do.
It is easily granted that I can imagine that there are such things as immaterial entes. Moreover, I can imagine a kind of “ensness” that all entes participate in. However, just because I am able to imagine such things, it does not follow that they be true concepts. Now that this “new” kind of ens has been discovered, i.e. immaterial ens, it may be that a we will only ever be able to treat of such things as principles and never in themselves. Also, it is possible that our desire to have some account of ens which is beyond that of our common experience may likewise be beyond us. The formation of the concept of ens commune must be done very carefully if we are to uphold the rigors required of true philosophical inquiry.
In order to have an understanding of ens that is broader than material being, we must form a concept of ens that is not in matter and motion. The emphasis on the “not” follows St. Thomas' division of the sciences in his commentary on Boethius' De Trinitate. There St. Thomas makes a distinction between abstraction and separatio.7 In abstraction we consider one thing without reference to what it is in. As Aristotle says of the geometer,

He strips off all sensible qualities, e.g. weight and lightness, hardness and its contrary, and also heat and cold and the other sensible contrarieties, and leaves only the quantitative and continuous, sometimes in one, sometimes in two, sometimes in three dimensions, and the attributes of these qua quantitative and continuous and does not consider them in any other respect.8

Separatio, on the other hand, is considering one thing specifically as not in another. Such an activity belongs to the second act of the intellect which composes and divides and it is here that we find ens commune which is a conception of ens which is not in matter and motion. It is important to note here, again, the importance of carefully and rigorously forming this concept. The second act of the intellect is where truth and falsity is first found. We do not have the assurance of the inerrant first act of the intellect. The possibility of error is therefore quite real.
It is going beyond the scope of this paper to extensively defend the truth of ens commune. Looking to Aristotle as an example, it seems that to have certainty in making such a judgment we need to reflect long and carefully over our experience. As was mentioned above, the formation of this concept and the feasibility of the science of metaphysics as a whole is in question for Aristotle into the eleventh book of the Metaphysics. Therefore, we will at this point proceed, assuming that this judgment is true. It remains, therefore, to see how what has been said of ens commune illumines the initial claims made by St. Thomas, namely that it is a genus and the subject of metaphysics. It seems clear that ens commune cannot be a genus in any strict sense of the term. Ens can in no way be a genus, since it is said equivocally of various categories. A fortiori, ens commune will not be said univocally. However, in St. Thomas' De Veritate he makes an argument for a broad sense of genus.9 He justifies his claim with the authority of Scripture, where God and humans are said to be in a genus.10 A fortiori, we are able to use genus loosely when speaking of ens commune.
To fully justify this particular use of genus, it seems necessary that there be some basis for calling ens commune one. It would be unfitting to place any chance grouping under a genus, even said in a broad sense. If there is no basis in the things, then anything can be a genus. Therefore, it seems clear that there must be a basis in things for calling something a genus. It would seem to follow then, that if there were some way in which something could be understood to be as one, even as it was said of many, that then there would be a basis for calling it a genus.
Aristotle, as has been said, concerns himself with the question of the unity of the subject of metaphysics in book eleven. Aristotle first manifests that, "everything that is may be referred to something single and common."11 However, at this point it seems that "is" is said in an equivocal manner. Aristotle goes on to argue that the contrarieties also may be referred to the first contrarieties of being. However, every pair of contrarieties is examined by one science. Then, he rules out the other sciences as in sufficient to treat of ens qua ens, all of them being concerned with ens under a particular account. It remains, therefore, to metaphysics to treat of ens qua ens. Having examined the nature of contrarieties, and seen how they are also referred back to something one and common, "since all that is is said to be in virtue of something single and common." It would seem therefore, that what is single and common is substance. For all accidents exist and are entes through substance. Therefore, in so far as something is an ens, so it must belong to substance, either as substance, or as an accident, or the contrarieties. Therefore, we have found a basis for considering ens commune as one.
This conclusion seems to grant further insight into how we should understand common in ens commune. It seems here that we should understand it as a kind of formal cause, for ens commune is the account given of what makes something to be an ens. This cause, however, should not be understood as external to the thing, but rather as an intrinsic cause. Here we may draw an analogy to the concept of animal. Animal may likewise be seen as an intrinsic formal cause, since it belongs to the account of what it is to be something, but cannot be found as separate from the account of such things either. This would seem to be the case for other genera as well.
From what has been said, there appears to be grounds for considering ens commune as an ens ratione. For the account of why it is common seems to belong more to our conception than to things themselves. However, there is a distinction among entes rationes. There are some which are without a foundation in things and some that do have a foundation in things. Non-being is an example of an ens ratione without a foundation in things. It is, as it were, the exemplar of non-existence. It is a negative concept which we conceive of in the mode of substance, but is not directly based on any experience. Time, on the other hand, is an ens ratione with a foundation in things. Time, according to Aristotle, is "the number of motion according to before and after." Consequently, it is a being of reason since it depends on there being some mind for it to exist. However, it is not without a direct foundation in things, since it is measuring something real viz., motion. Ens commune therefore seems to belong to those entes rationes with a foundation in things. For ens is most of all real, though the account of it being common belongs to our conception. Therefore, ens commune is an ens ratione with a foundation in things.
The mode of this paper has been largely dialectic in an attempt to manifest the course of thought that is required for the formation of the concept of ens commune. Many of the conclusions, therefore, do not necessarily conclude with complete certainty. It does seem clear from what has been said, that ens commune is an attempt by our mind to find a broad account of ens which is not restricted to matter. Moreover, it seems clear that if such a concept is formed that it can be based on a loose notion of genus. Finally, the account for its unity, though briefly and summarily given, seems to provide sufficient grounds for calling it one. What is left quite uncertain by this paper is how ens commune can be the subject of metaphysics. That ens commune is an ens ratione, likewise lacks to certainty that one would require before agreeing to such a claim. These two claims are quite involved with one another. If ens commune is an ens ratione, then it seems difficult to see how it is fitting matter for a science. Likewise, the grounds for calling it common and an intrinsic formal cause are left uncertain, since these were used to arrive at ens commune being an ens ratione.

Notes:
1 Metaphysics 1059a21
2 Physics
3 Super De Trinitate Q. 5 a. 4
4 The english word “being” being ambiguous in its usage, compels me to therefore use ens throughout this paper in order to signify an an entity.
5 De Anima 430a 23
6 Physics 267b 24-25
7 Super De Trinitate Q.5 a3
8 Metaphyiscs 1061a 30-36
9 De Veritate Q.1 a. 4 ad s.c. 8
10 17:28 The Vulgate reads thus: ipsius enim et genus sumus
11 Metaphysics 1061a10