The Main Branches of Philosophy are divided as to the nature of the questions asked in each area. The integrity of these divisions cannot be rigidly maintained, for one area overlaps into the others.

1. Axiology: the study of value; the investigation of its nature, criteria, and metaphysical status. More often than not, the term "value theory" is used instead of "axiology" in contemporary discussions even though the term “theory of value” is used with respect to the value or price of goods and services in economics.
   1. Some significant questions in axiology include the following:
      1. Nature of value: is value a fulfillment of desire, a pleasure, a preference, a behavioral disposition, or simply a human interest of some kind?
      2. Criteria of value: de gustibus non (est) disputandum (i.e., (“there's no accounting for tastes”) or do objective standards apply?
      3. Status of value: how are values related to (scientific) facts? What ultimate worth, if any, do human values have?
   2. Axiology is usually divided into two main parts.
      1. Ethics: the study of values in human behavior or the study of moral problems: e.g., (1) the rightness and wrongness of actions, (2) the kinds of things which are good or desirable, and (3) whether actions are blameworthy or praiseworthy.
         1. Consider this example analyzed by J. O. Urmson in his well-known essay, "Saints and Heroes":
            
            "We may imagine a squad of soldiers to be practicing the throwing of live hand grenades; a grenade slips from the hand of one of them and rolls on the ground near the squad; one of them sacrifices his life by throwing himself on the grenade and protecting his comrades with his own body. It is quite unreasonable to suppose that such a man must be impelled by the sort of emotion that he might be impelled by if his best friend were in the squad."
         2. Did the soldier who threw himself on the grenade do the right thing? If he did not cover the grenade, several soldiers might be injured or be killed. His action probably saved lives; certainly an action which saves lives is a morally correct action. One might even be inclined to conclude that saving lives is a duty. But if this were so, wouldn't each of the soldiers have the moral obligation or duty to save his comrades? Would we thereby expect each of the soldiers to vie for the opportunity to cover the grenade?
      2. Æsthetics: the study of value in the arts or the inquiry into feelings, judgments, or standards of beauty and related concepts. Philosophy of art is concerned with judgments of sense, taste, and emotion.
         1. E.g., Is art an intellectual or representational activity? What would the realistic representations in pop art represent? Does art represent sensible objects or ideal objects?
         2. Is artistic value objective? Is it merely coincidental that many forms in architecture and painting seem to illustrate mathematical principles? Are there standards of taste?
         3. Is there a clear distinction between art and reality?
2. Epistemology: the study of knowledge. In particular, epistemology is the study of the nature, scope, and limits of human knowledge.
   1. Epistemology investigates the origin, structure, methods, and integrity of knowledge.
   2. Consider the degree of truth of the statement, "The earth is round." Does its truth depend upon the context in which the statement is uttered? For example, this statement can be successively more accurately translated as …
      • "The earth is spherical"
      • "The earth is an oblate spheroid" (i.e., flattened at the poles).
      • But what about the Himalayas and the Marianas Trench? Even if we surveyed exactly the shape of the earth, our process of surveying would alter the surface by the footprints left and the impressions of the survey stakes and instruments. Hence, the exact shape of the earth cannot be known. Every rain shower changes the shape.
      • (Note here as well the implications for skepticism and relativism: simply because we cannot exactly describe the exact shape of the earth, the conclusion does not logically follow that the earth does not have a shape.)
   3. Furthermore, consider two well-known problems in epistemology:
      1. Russell's Five-Minute-World Hypothesis: Suppose the earth were created five minutes ago, complete with memory images, history books, records, etc., how could we ever know of it? As Russell wrote in The Analysis of Mind, "There is no logical impossibility in the hypothesis that the world sprang into being five minutes ago, exactly as it then was, with a population that "remembered" a wholly unreal past. There is no logically necessary connection between events at different times; therefore nothing that is happening now or will happen in the future can disprove the hypothesis that the world began five minutes ago." For example, an omnipotent God could create the world with all the memories, historical records, and so forth five minutes ago. Any evidence to the contrary would be evidence created by God five minutes ago. (Q.v., the Omphalos hypothesis.)
      2. Suppose everything in the universe (including all spatial relations) were to expand uniformly a thousand times larger. How could we ever know it? A moment's thought reveals that the mass of objects increases by the cube whereas the distance among them increases linearly. Hence, if such an expansion were possible, changes in the measurement of gravity and the speed of light would be evident, if, indeed, life would be possible.
      3. Russell's Five-Minute-World Hypothesis is a philosophical problem; the impossibility of the objects in the universe expanding is a scientific problem since the latter problem can, in fact, be answered by principles of elementary physics.
3. Ontology or Metaphysics: the study of what is really real. Metaphysics deals with the so-called first principles of the natural order and "the ultimate generalizations available to the human intellect." Specifically, ontology seeks to indentify and establish the relationships between the categories, if any, of the types of existent things.
   1. What kinds of things exist? Do only particular things exist or do general things also exist? How is existence possible? Questions as to identity and change of objects—are you the same person you were as a baby? as of yesterday? as of a moment ago?
   2. How do ideas exist if they have no size, shape, or color? (My idea of the Empire State Building is quite as "small" or as "large" as my idea of a book. I.e., an idea is not extended in space.) What is space? What is time?
   3. E.g., Consider the truths of mathematics: in what manner do geometric figures exist? Are points, lines, or planes real or not? Of what are they made?
   4. What is spirit? or soul? or matter? space? Are they made up of the same sort of "stuff"?
   5. When, if ever, are events necessary? Under what conditions are they possible?