Another aspect of the depth of being is the diversity of being. Tillich develops many themes of meaning, diversity, and historicity in laying out the Gospel framework and translating it into his phenomenological take on the diversity of being. Human being, fallen nature, sin, redemption, new being in Christ, these are standard Christian themes but a good deal of his Systematic Theology is devoted to exploring them from the perspective of their relationship to being. What he’s doing there is demonstrating the depth of being ontologically and in terms of human experience (vol II of Systematic Theology). Volume I of that work is about “Being and God.” Here he deals with topics of “The Question of Being: Man, Self and World.” “God is the answer to the question implied in being” he says.  He first deals with reason and revelation. Then he moves into the question of being and its meaning. He says that in coming to terms with reason and its take on existential conflicts, one is forced into asking the most essential question of all, why is there something rather than nothing at all? But I have given this in Heidegger’s terms. Tillich puts it a bit differently “why is there something, why not nothing?” He points out that to ask “why is there not nothing?” is to attribute a kind of being to nothingness. Thus as he puts it “one cannot go behind being.” What he’s saying is, like trying to imagine one’s own non existence, it can’t be done. We cannot get under being itself, its’ the furthest we can go back in our understanding, and it eludes our understanding. Thought is based upon being and it can’t go beyond its base. One can imagine the negotiation of things, however, and it can “describe the nature and structure of everything that is the power of resisting non being.” Ontological questions, he points out, are not tautologies because of this ability to mentally play with being and non being. We are not merely saying “being is being” when we try to define what it is, because there’s a possibility of negating any particular form of being. The possibility of universality and less than universal aspect of forms of being make ontology possible. There are concepts which are less universal than being but more universal than any concept about being, thus these are “categories” of thought.
/...These categories form the basis of theological significance. These are central concepts that make theology “go,” so to speak (not Tillich’s phrase). These are ontological concepts, ontology is not theology. One can be an atheist and totally secular and do ontology as part of philosophy, and such a thinker would have to deal with these concepts. But in like manner all theologians must deal with them as well. While they are not theology per se they are essential to theology. The concepts are: (1) the structure implicit in the basic ontological question (why is there something rather than nothing?); (2) the elements which constitute ontological structure; (3) characteristics of being which are the conditions of existence; (4) categories of being and knowing.  The structure (1) is that the question presupposes an asking subject, and an object being asked about. This is the subject/object structure that is presupposed and that in turn assumes the structure of world and self; this as the basic articulation of being. That the self has a world to which it belongs and from which it will deduce the nature of its being precedes all other structures and will be the basic analysis which precedes all other analysis.  The elements of the ontological structure he groups into three sets of pairs: individuality and universality, dynamics and form, and freedom and density. These are polarities and the first expresses self referential nature of being.
The ontological concepts pertaining to number (3) (characteristics of being) “expresses the power of being to exist,” in Tillich’s own words, “and the difference between essential and existential being.”  There is a duality for Tillich between essential and existential thinking. One is inherent in the other, as existentialism is meaningless without an essentialism to play off it. No ontology can disregard these two aspects.  Existentialism is a revolt against the predominance of essentialism. Essentialism came to be identified in theology with “stasis” and existents with movement, or process theology. Tillich saw a unity between the two, one assuming the other. Tillich says essentialism is related to universalism, and we can’t deal with concepts in the world without universals. Thus existentialism has to assume essentialism and the two have to work together. The fourth level deals with the categories of thought or the basic concepts. These he calls “structures of finite being and thinking.” I suppose the Kantian categories would be placed here. “If time and space are called ‘categories’ this is a derivation from the Kantian terminology which calls time and space forms of intuition. But the larger sense of category has been accepted generally, even in post Kantian schools.” Tillich says that determining the exact nature and number of these categories is the on going and never ending task of philosophy.  He isolates four such categories: time, space, causality, and substance. These are categories that have the most theological importance. Quantity and Quality he says have less theological importance. He discusses other categories and their relation to the four points above, but I will forgo that as it really doesn’t have a direct bearing on the task before us here. He does focus on finitude at this point (p165) as having a major bearing on the ontological question of God.
....He’s going to argue that ontological concepts are a priori. What he means by a priori is not quite the same as most logicians understand it. We think of a prori as a tautological statement, a statement where we only need to know the meaning of the terms in order to understand the truth of the statement. Tillich makes it sound like the thinks a prori means empirical data. He says it’s ultimately a matter of experience. I don’t think he’s confusing it with empirical data. He is saying that the ultimate understanding of what terms mean is a matter of experience. In other words we think of a prori as statements like “all husbands are married men.” If we know what a husband is we know all of them are married men. Tillich is saying that the idea of husbands and marriage is not some eternal truth in a vacuum. We only have a concept of those terms because we live in a culture that has a convention of marriage. Thus in an ultimate sense the a priori concepts originate form the experience of a life world in which cultural constructs have a shared meaning. The concepts of Being, the categories, are a priori but in the same way rooted in our experience of being. As Tillich says “they constitute the very structure of experience itself.” IF experience changes a new a priori will from. Tillich discusses process theology and the question of a static understanding of God. He identifies with a tradition from Scotus to Heidegger, picking up Bergson along the way, and moving toward indeterminacy in the ground of being. But it dose not remove a prori structure from ontology or Being.
Still setting up the discussion of finitude and being, he moves to the prelude to that discussion, the self-world relationship. Every being participates in the structure of being, but man alone (in so far as we know) is aware of it. We are the only being we know that has alienation and estrangement. We can describe behavior but we do not know what the behavior means to others. We are the only being we know of that asks the ontological question (why is there something rather than nothing?) and the only one that can try to answer it. In Heideggerian terms, as Tillich puts it, we are only able to answer because we understand the nature of “being there.” Or Tillich speak, we experience “directly and immediately the structure of being and its elements. As stated above the ontological structure is the structure of the ontological question, the assumption and self and world, and that’s what we are moving to as a prelude of discussion of finitude. Then there is also no 2 from above the structure of being grouped into three sets of pairs: individuality and universality, dynamics and form, and freedom and density. These are polarities and the first expresses self referential nature of being. These are a prori concepts. Self and world is a basic part of this structure. Humanity is not merely a passive object of study, but a living consciousness in the process of learning and apprehending these structures first hand. Humanity cannot be turned into an object of study under the guise of making understanding easier. We are the student as well as the object, so to reduce humanity itself to an object is lose the phenomena of what it means to experience being the object or being thing studied. We can’t step outside of that experience and study it as an object dispassionately without changing our understanding of what that thing is we would study. This leads into what Tillich discusses in The Courage To Be where speaks of the courage to be a part of and the courage to be apart from.
As the ontological question implies humanity understands itself as having selves that live in a world. This is the organically a priori set up of asking the question. The relationship between self and world is dialectical, we must be a part of, and we must be apart from. To study, to understand to live, to know, to remain true to what we understand we must go play this game of tag, now standing alone as apart from the world, now standing with the world as part of it. There is no question of the existence of the self, according to Tillich. The Postmodernists made a big deal out of the idea there is no core self. That is a somewhat different question, however, depending upon what is meant by “core,” but there is clearly some form of self since someone had to write those articles, and since even making the argument “there is no self” would require that one be a self and understand something about the concept. According to Tillich the question is self awareness of self relatedness. This is a dialectical relationship in another way as well, in that the relationship of self and world is part of the larger dialectic of being and nothingness, because it is part of the depth of being and part of the basic categories that emerge from ontological structure. So the importance of this is going to be that in the discussion of finitude the apprehension of our own finitude and what we make of that vis a vi Being itself and it meaning in terms of the object of ultimate concern is hinged upon self understanding, and understanding of self in relation to the world as a crucial aspect of the depth of being; thus this will figure into understanding being itself as indicative of the object of ultimate concern. As shall be seen the object of ultimate concern is indicative of the divine aspect of Being itself, or “holy being.”
The self world polarity is the basis of the subject/object structure of reason, according to Tillich.  The world is seen as a structured whole, as such it is called “objective” because the many self-world relationships in being all relate more or less the same basic idea of a world. The self is a structure of “centeredness” in terms of awareness, for this reason it is termed “subjective.” In other words subjective refers to the center of awareness which takes in the sense data and relates itself to that which is beyond itself, the world. Objective refers to the single “outside” nature of that which is shared in this awareness by the many selves. Reason is actually makes these, that is it makes the self a “self” and the world a “world.” This is because it is through our constructs of reason that we attach meaning to these terms and understand them in relation to each other, which is a function of their structured relationship. Without the structuring aspect of reason being would be chaos. “Where there is reason there is a self and a world in interdependence.” In cognitive terms anything toward which the cognition is directed is considered an object, be it God, or individual items in nature, attitudes, or ideas. We cannot resist making God an object for this very reason. If we think about the concept of God we make God an object. This holds a danger, however, in that we tend to objectify that which we hold in this act of cognition. “If God is brought into the subject-object structure of being he ceases to be the ground of being and becomes one being among others (first of all a being beside the subject who looks at him as an object). He ceases to be the God who is really God. “ Various theologies try to escape this problem in various ways. The prophetic tradition insists that we cannot see God; sight is the most objectifying aspect of cognition. Knowledge of God is reveled and understood through man, thus even when God becomes the object God remains the subject (this is just how Tillich puts it). Mysticism attempts to overcome the problem by ecstatic union. In whatever way the resolution is achieved it must be to acknowledge that no language of God can make God an object. Thus language about God must be either negative, or analogical.
There is another sense in which something is made into an object, according to Tillich, that is in robbing it of all of its subjective elements. That is, to turn something into a “thing.” We resist calling human beings “things” because our subjective qualities lead us to disvalue mere things as inhuman, and to value humanity because of its subjective elements.  One of Tillich’s major concerns is that God not be treated as a “thing.” For those who believe that Tillich is reducing God to the level of an impersonal force or mere abstraction this is another rebuff. But atheists reduce God to the level of a thing, and turn God into another thing in creation alongside all the many things we see in the world. This has nothing to do with personality but it does mean God can’t be conceived as just an impersonal force or a mere abstraction without defeating Tillich’s purpose. He does not include this argument, but it seems rather clear from what he says. The reductionistic atheist reduces all things to the level of “a thing” devoid of subjective elements. Atheists greatly fear subjectivity. That’s always the bottom line in all of their refutations of God arguments, “that’s subjective.” The reductionist view-point treats all sense data as “information” and information is a collection of things, which can be homogenized and abstracted into “data” and “reduced” to it’s most basic level which of course would lose any subjective elements as it loses the phenomena that makes the aspect that which requires reducing to fit into the atheist world view. The reductionist sees human perceptive powers and thought as side effects of chemicals and brain function that makes thought “mere subjectivity” and that is among the phenomena to be lost in explaining human consciousness. To reduce humanity to “a thing” one must reduce human consciousness to a mere epiphenomenon. Parmenides saw the basic ontological structure as the unity of being and the word (logos) in which it is grasped. Thus from this Tillich draws the observation that subjectivity is not an epiphenomena but a primary phenomena although related in polar opposite to objectivity. One cannot derive subjectivity from objectivity or vice versa. The attempt to do so has meant either the subjugation of humanity to numbers and to machines, or the romantic rebellion and undisclosed abandon which sacrifice reason. Tillich asserts that the basic ontological structure cannot be derived. The relation is one of polarity. “What precedes the duality of self and world, of subject and object,” he asks? His answer is that this is a question in which “reason looks into its own abyss—an abyss in which distinction and derivation disappear, only revelation can answer this question.” 
6] Tillich, ST I, 163.
6] Tillich, ST I, 163.
 Ibid., 163-64
 Ibid, 164
 Tillich, History…, op cit, 541.
 Tillich, ST 1, 166
 Tillich, ST I, 164.
 Ibid, 166
 Ibid, 168
 Ibid., 169-170.
 Tillich, Courage…, op cit, find
 Tillich ST I 169.
 Ibid., 171
 Ibid, 172
 Tillich, System I, 173
 Ibid, 174.