The second assumption of ontological philosophy is that ontology is a kind of explanation. Since ontology is the study of the nature of existence, that means explaining what is found in the world by the basic nature of what exists there. Traditional philosophy, by contrast, has taken ontology to be just a thesis about what exists, or realism about beliefs that are accepted on other grounds, such as empirical science.
To take ontology to be a kind of explanation is to hold that there are ontological causes and ontological effects and that the ontological causes produce the ontological effects.
The ontological causes are the basic substances postulated by an ontology and the basic relationships by which they are assumed to exist together as a world. Relationships must also be assumed, because effects are explained by substances working together in some way. Otherwise, the effects would be just ontological assumptions.
Substance is the first kind of ontological cause. Substances are traditionally thought to be entities of certain kinds that are self-subsistent. Their ability to exist on their own is what enables them to constitute the existence of what is found in the world.
The basic nature of substance as substance involves two aspects, an essential aspect and an existential aspect.
The essential aspect of a basic substance includes all the properties that distinguish it from other kinds of basic substances (and do not change). The properties of a substance are simply aspects of the substance itself. A substance must have an essential aspect to its nature as well as an existential aspect, because nothing can exist without existing in a determinate way.
The existential aspect of substance as substance is the fact that it exists. A substance must have an existential aspect to its nature as well as an essential aspect, because if it did not, there would be nothing to have the essential aspect of its nature. But there are two basically different aspects to the existential aspect of the nature of substance as substance.
Particularity is the fact that each substance has an existence distinct from the existence of every other substance in the world. Every substance is a particular substance, and together they make up the world.
Temporality is the fact that a substance as a substance, if it exists at all, exists at every moment in the history of the world. That is, basic substances do not come into existence and do not go out of existence over time. This is to assume that substances exist at a series of moments. But it is to leave open the question about whether existence itself is in time or time is a difference among parts of substances (that is, existence as endurance through time or as perdurance across time).
In order to explain everything in the world, the substances postulated by an ontology must be the most basic substances. There can be various kinds of basic substances, but there is a relevant deference in how their essential natures are defined: some possible kinds have temporally simple essential natures (exhibiting their natures completely at each moment they exist), and others have temporally complex essential natures (exhibiting their natures by how contingent properties change over time).
Basic relationships among substances are the second kind of ontological cause. Substances must have relationships of some kind in order to explain what is found in the world, because what makes ontological explanations genuinely explanatory is how substances combine to constitute the existence of things found in the world.
Relationships are not something that exist in addition to substances. They are, rather, aspects of substances, like the properties of substances, except that relations are aspects of the world, rather than substances taken separately. The basic relationships among basic substances are simply how those substances exist together as a world.
The relationships assumed by an ontology may be of various kinds, but there is a relevant difference in how they are defined: relations are temporally simple insofar as they are how substances exist together as a world at a single moment, and relations are temporally complex insofar as how they exist together as a world involves a regularity about how temporally simple relations change over time.
To take ontology to be realism, as epistemological philosophy does, is to take it to be just a thesis about what exists. Since the existence of whatever is used as the foundation for its epistemological argument is taken for granted, realism is belief in the existence of additional entities of some kind that are supposed to be known from that epistemological foundation. But since realism affirms the existence of two basically different kinds of entities, it finds itself committed to a form of ontological dualism that is problematic
The epistemological foundation of ancient philosophy was the belief that perception and reason are both forms of intuition that disclose different kinds of objects. Thus, in addition to perceivable objects, Plato argued for the existence of Forms in a separate realm of Being, and Aristotle argued for the existence of essential forms in addition to matter. In either case, it was realism about forms.
Realism in medieval philosophy was belief in the existence of God as the creator of the natural world, and the epistemological foundation of its arguments for the existence of God was what is known about the natural world. Hence, the basic form of realism was supernaturalism. (But when the natural world was assumed to include essential forms, as well as matter, it included ancient realism as well.)