This is the initial post in a series on Thomistic metaphysics. The goal of the series as a whole, and of this post individually, is to introduce readers to a basic understanding of Thomas Aquinas’s conception of metaphysics and to show the connection of this with theological considerations, whether it be with reference to God, man, etc. Before considering specific aspects of Thomistic metaphysics (act-potency distinction, esse-essentia distinction, etc.), there are two preliminary remarks that need to be made regarding that which will comprise this post: 1) a definition of metaphysics from the Thomistic perspective must be presented and 2) a brief word on the relation of metaphysics (and of philosophy more generally) to theology.
First, what is metaphysics?  Bernard Wuellner, in his Dictionary of Scholastic Philosophy, defines metaphysics as, “the science of the absolute first principles of being.”  That is, metaphysics is that science which deals with those aspects of reality that transcend all others, “behind” which there is nothing left to consider. This is what is meant by “first principles,” namely, that which “does not proceed from a prior principle,”  or that which is most fundamental to existent reality. For example, there is the first principle of essence – that is, that by which a thing is what it is; when we speak of essence we are here considering the “humanness” of a human, or that by which a human is a human. These are the sort of things that are considered in metaphysics. Thus, all other considerations of reality (i.e., physics, biology, chemistry, psychology, etc.) stem from the considerations of metaphysics; this is why a common definition of metaphysics is ‘considerations of ultimate reality.’ For clarification, in Wuellner’s definition, “being” refers to that which exists. Metaphysics is not concerned with specifics between mammals and amphibians, but with the distinction and categorization of various aspects of those beings according to the previously mentioned “first principles.” Thus, Ralph McInerny, in his introduction to a volume of Thomas Aquinas’ selected writings, says, “there is a theoretical science that Aristotle called theology or first philosophy, and that came to be called metaphysics…. metaphysics was assigned being as such, or being as being, as its subject.”  The definition that McInerny presents, namely, being as being, is my prefered definition, since it clearly denotes that it is being itself and its various aspects that is the subject of metaphysics.
Metaphysics and Theology
Second, what is metaphysics’ relationship to theology? Generally among Protestants, a hard-fast distinction is made between metaphysics (or philosophy, more generally) and theology, such that the two are either proposed to be entirely distinct disciplines with very little significant overlap or to be contrary disciplines that oppose one another. These views are, plainly put, inadequate; no one can escape the reality of philosophical influences. Nor can one deny the influence of philosophic ideas upon any given theological tradition. It is in light of the inadequacy of these views that we must consider the relationship of philosophy, particularly that of metaphysics, to theology. This relationship is one which is currently under debate within Reformed circles—such as in the recent controversy centering around James E. Dolezal’s All That Is In God and theology proper. I believe that Ralph McInerny excellently summarizes the relationship of metaphysics and theology:
“The whole aim of philosophy, as it was begun by the Greeks, is to achieve wisdom; wisdom is knowledge of first principles and causes; but the first principles and causes are divine. Philosophy by definition strives towards knowledge of the divine, and if it is successful, ends as theology. Truths about God do not begin where philosophy ends; they are the telos of the whole philosophical enterprise…. This is what made it wisdom, according to Aristotle, and that is the goal of philosophy.” 
Metaphysics, as considering the first principles of being, or being as being, necessarily leads to natural considerations of God insofar as the first principles of being have no principles above them from which they come and, thus, on which they depend. For God is Himself is absolute being in the sense that He is utterly independent of all else to be Himself; this idea is usually described by the philosophical-theological shorthand of God’s aseity, or God as a se. God does not depend upon principles of being in the way creatures do, rather these are used to describe Him that we might rightly understand God in Himself and all else in relation to Him. Therefore, God is the ground of being upon which all creaturely being depends and because of that any consideration of metaphysics must culminate in consideration of the divine or else the subject has only been shallowly investigated.
 Throughout the history of philosophy, various definitions of Metaphysics, with differing levels of precision have been presented. To remain intellectually honest with my readers, I must admit that the definition which I present is one of many potential definitions. Nevertheless, in my estimation, this is one of the best definitions.
 Bernard Wuellner, Dictionary of Scholastic Philosophy (Fitzwilliam, NH: Loreto Publications, 2012), 76.
 Wuellner, Dictionary of Scholastic Philosophy, 97.
 Ralph McInerny, ed. “Introduction” in Thomas Aquinas: Selected Writings (New York: Penguin Books, 1998), xvi.
 McInerny, ed. “Introduction” in Thomas Aquinas: Selected Writings, xv.