A thing in simplest terms is nothing else but a being, that is, one that exists. However, note that there are two kinds of beings. First, there are real beings, that means, beings that exist outside the mind. Most of what we call “things” are, in fact, real beings. Second, there are beings of reason, or beings that can’t exist, except only in the mind, such as, for example, the idea of a “mermaid”, a “square circle”, or even “evil.” All these are also “things” because they, too, exist, but only in the mind as beings of reason (entia rationis). Note that we can’t think of anything at all, or talk about anything, unless we think of it first as a being. So, if a thing does not exist in reality, the mind itself gives it its “thingness” or its intelligibility, by giving it a purely mental or intellectual existence.
What about “nothingness.” Is it a “thing”? Sure it is. It is a concept in the mind that represents the state of non-being. So, nothingness is also a thing, but it is a being or reason, not a real being. There is no such thing as a real nothingness existing in the world. If there was a time before God created anything in the world, then there was a time when outside of God there was absolutely nothing. Could we then say that outside of God, before the creation of the world, there was an entity, called “nothingness”? Nope! Because nothingness is not a real being or a real entity. Before God created anything, there was nothing that exists outside Himself. There was no time, no empty space, and no nothingness either!
Let me dig this a little more. Real beings are of two kinds: actual beings and possible (or potential) beings. Actual beings are real beings that exist in reality here and now. Possible or potential beings are real beings that do not exist in the real world, but which can exist. They are distinguished from beings of reason because, although they do not exist here and now, they have a reality in the Mind and Power of God. A city with 1000 waterfalls may not be an actual being, but it is in the power of God to create; so it is a real, possible being. Things that have a contradiction in their notion, such as a “square circle” are purely beings of reason, and are not possible beings at all. Not even the infinite power of God can make a being of reason of this kind real.
However, do all beings of reason involve a contradiction? Not at all. For example, there are mathematical entities, such as the square root of -1, that do not exist except in the mind, but which do not involve an inherent contradiction. These are beings of reason, but not possible beings. Philosophers have a special terminology for them. They are called “beings of reason with a foundation in reality,” entia rationis cum fundamento in re.