Method. The final assumption needed to secure a foundation for ontological philosophy is a method for deciding which of the possible ontological explanations to believe. We will assume that we ought to believe the best ontological explanation of the world, and since we are naturalists, that means preferring the best ontological explanation of the natural world. Since the empirical method can be defined as inferring to the best explanation, that makes the foundation of ontological philosophy empirical ontological naturalism.
The empirical method is the same method that science uses, except for applying it to a different kind of explanation. But it is not the only possible method for deciding what to believe. The alternative is the rational method of traditional, epistemological philosophy. Its foundation was a theory about how we know, which was based on reflecting on our processes of knowing. It might also be considered an inference to the best explanation. But since the way we ordinarily explain what is known by reflection is by giving reasons, the method of epistemological philosophy always came down to the claim that certain truths are required by reason itself. Though the actual standard was different in different eras of Western philosophy, they can all be called forms of the rational method.
The empirical method, by contrast, may be considered an inference to the best explanation of what is known by perception. Perception provides relevant evidence in deciding what to believe because it discloses facts about what exists in the world. But for naturalists seeking an ontological explanation, there is no need to limit the evidence to perception.
Given our assumption, as naturalists, that the natural world is the world disclosed to us by perception, the empirical method might also be described as inferring to the best explanation of the natural world. Though science may limit itself to explaining what is known by perception, the latter formulation is preferable, given our ontological purposes, because there is no need to limit the evidence we have about the natural world to what is known by perception. Reflection should also be accepted as providing evidence about the nature of the substances and relations constituting the natural world, because we believe, as naturalists, that the beings in whom reflection occurs are themselves parts of the natural world. That would not be to revert to the rational method of epistemological philosophy, as long as we take reflection and what is known by it to be something found in the natural world that needs explaining, and not as providing a standard for judging what is true. What is known by reflection is no less evidence of what exists in the natural world than what is known by perception, though when we define "naturalism" ontologically, as holding that the world is just what is in space and time, we are taking perception to disclose its basic nature more completely. Thus, since it is the natural world itself, not just what is perceived, that we are trying to explain ontologically, we shall interpret the empirical method broadly as inferring to the best explanation of the natural world, not just what is known by perception.
Having assumed naturalism and the validity of ontological explanation, the third and final assumption of ontological philosophy is the empirical method. That is, if this argument is logically valid, it will not be possible to reject the necessary truths justified by it, unless one denies naturalism, the validity of ontological explanation, or the empirical method.
The empirical method. By the "empirical method," I mean an inference to the best explanation of what is found in the natural world (either by perception or perception and reflection). Though this way of deciding what to believe presupposes a kind of explanation, the method can be stated abstractly, because its standard for judging what is best that can be applied to any kind of explanation, or at least, any kind that cites causes in order to explain effects. So let us consider the method abstractly, and then take up the various applications of it.
Inference to the best explanation of the natural world. The standard for the best explanation is simply explaining the most with the least. The best explanation can be identified as the one that requires the least in the way of causes to explain the most in the way of effects. After explaining what this empirical standard requires generally, we will see how it applies to various kinds of explanation, including ontological explanation.
Scope. The explanation with the greater scope is better, other things being equal. That is, if two explanations are equally simple, the empirical method requires us to prefer one over the other, if it explains more of what is found in the world than the other.
The preference for explanations with larger scopes does not always determine which explanation to believe even when other things are equal. When two theories have overlapping scopes, for example, it may be unclear which explains more.
Simplicity. The simpler explanation is better, other things being equal. What does the explaining in an explanation are its causes, for they produce the effects, which are what is explained by the explanation. Thus, if two explanations explain the same range of phenomena, the empirical method requires us to believe one rather than the other, if it requires fewer causes or the causes it requires are simpler.
Nor does the preference for simpler explanations always determine which theory to believe when other things are equal. There may be a trade-off between fewer causes and simpler causes. There is no way to say in general whether to prefer fewer, more complex causes or a larger number of simpler causes. It depends on the kind of explanation involved or, perhaps, the specific case. And even then, there may be no way to decide.
Scope and simplicity are the basic criteria for judging explanations, but there is no reason to deny that there may be other issues about which is the best explanation that arise when specific kinds of explanations are being considered. Appeal can always be made to the basic standard for judging the best among explanations of the same basic kind: explaining the most with the least.
Two sources of error using the empirical method should be noticed.
First, any limitation in the range of theories being considered can lead to errors. Since the empirical method chooses the best among the possible explanations, it works only insofar as all possible theories are being considered.
Second, any limitation in the range of evidence being considered can lead to errors. Since the empirical method chooses the best explanation of what is in the world, it works only insofar as we have found everything relevant in the world. And as mentioned above, naturalists have no reason, in principle, not to include as evidence, along with perception, what is found out about the natural world by reflection, if it is relevant. The subjects and the mental processes on which they reflect are part of the natural world.
Kinds of inferences to the best explanation of the natural world. Since the empirical method is relative to the kind of explanation being sought, we must have the ability to comprehend some kind of explanation in order to use it. Nor can we say in advance which kind of explanation ought be used. We must simply develop whatever ways of explaining we can understand, and then compare them to see how they fit together or, if we must choose among them, which to believe.
Efficient-cause explanations. The empirical method of science is to infer to the best efficient-cause explanation. Explanation by efficient causes is understood as depending on laws of nature, which describe regularities about how causes lead to effects. It is usually represented by the deductive-nomological model (or covering law model, which can be traced to David Hume). This model holds that an event (or regularity) is explained when a description of it can be deduced from true laws of nature and the relevant initial and boundary conditions.
The initial and boundary conditions, or certain salient parts of them, are said to be the cause, and the event (or regularity) entailed by them and the law of nature is the effect.
This model works well for physics, but there has been a long dispute about its adequacy for other branches of science. Those disputes are not relevant here, since we are more concerned with comparing efficient-cause explanations with other forms of explanation than with details about how it is applied in specific cases. (A better account of the kinds of scientific explanations that this model slights will be given when we take up the necessary truths of ontological philosophy. See Change: Epistemological theories of causation)
Scope. The explanation of any specific event (or regularity) is just one of a whole range of explanations that may be based on the same law, and the scope of the explanation includes all the events (and regularities) that can be explained by it. According to the empirical method, therefore, the best efficient-cause explanation, other things being equal, is the one that follows from the most general laws of nature, that is, the natural laws with the largest scope.
The explanation with the fewest causes, in the case of efficient-cause explanations, would be the one with the fewest relevant initial and boundary conditions. Since what makes such conditions relevant are the laws of nature, this is usually the requirement of preferring efficient-cause explanations that require the fewest laws. Thus, given any two explanations with the same scope, the empirical method requires us to prefer the one requiring the fewest laws of nature and the fewest relevant initial and boundary conditions. But if two explanations appeal to the same laws, we should prefer the one that requires the fewest and simplest initial and boundary conditions.
The explanation with the simplest causes may also mean, in the case of efficient-cause explanations, the one with the simplest laws of nature. The criterion of simplicity in this case has notorious problems, because natural laws formulated in terms of quantitatively precise mathematical formulas can be simple in different ways. However, even without a generally accepted standard of mathematical simplicity, scientists usually manage to reach agreement on this matter. Those issues need not, in any case, concern us, given the altitude of our comparison of these forms of the empirical method.
Since criteria for explaining the most with the least can be traded off against one another, the empirical method does not necessarily determine which theory to believe in science. But this is how the goal of science is usually formulated. The so-called "holy grail" of contemporary physics is an example. That goal is to find a single, basic natural law that would cover all the forms of motion and interaction among bits of matter that physics recognizes, including not only electromagnetism and the weak and strong (or color) forces, but also gravitation. This goal shows a commitment to finding the simplest explanation with the largest scope, though physicists have encountered intractable problems in their quest to formulate such a law. (The biggest problem is that it does not seem possible to state Einstein's theory of gravitation in the same kind of mathematical formulation as the laws for the other basic forces, that is, as a quantum field theory, without postulating ten or more dimensions of space!)
Efficient-cause explanations are also given in ordinary life, engineering, and less basic branches of science, where the empirical method is applied more loosely. We can understand most causal connections apart from formal deductions for mathematically formulated laws of nature, because we have a form of imagination (spatial imagination) that enables us to think about the relations of objects in space and to how they change as objects move and interact over time. Spatial imagination represents very basic regularities, which are implicit in the laws of physics, but it can also represent what specific laws of nature require against this background understanding. This remarkable capacity is easily overlooked, because it is built into our faculty of perception as our way of understanding what perception discloses about nature. In any case, this way of understanding efficient-cause explanations enables us to use the empirical method, because, despite its non-formal nature, it enables us to see which theory explains the most with the least.
When events that depart from expectations, such as accidents, for example, are explained by efficient causes, the empirical method enjoins us to prefer the explanation that requires the simplest causes (the simplest deviations from normal, which are most likely) and the fewest causes (rather than a combination of independent deviations). But it also requires us to prefer the explanation with the largest scope, and thus, we prefer an explanation that can also account for other details about the accident. Or in the case of regularities generated by a mechanism of some kind, the empirical method would have us prefer the simplest mechanism that can explain the most about the regularity in its behavior. Such judgments depend more on our capacity for spatial imagination than precise formulations of laws of nature, though the latter may be relevant in choosing among them when more precise quantities are relevant.
Rational-cause explanation. Though social science also uses the empirical method of natural science, it has another kind of explanation which it shares with the humanities, distinguishing it from natural science. It is called "rational explanation." Since it explains phenomena by causes, the empirical method can be used in inferring to the best rational explanation. But the nature of rational explanation is such that the empirical method does not, in general, lead to agreement about what to believe about the world. What follows is not meant to defend rational explanation in science, but merely to show how rational explanation can be seen as another instance of the empirical method.
It is possible to explain what rational beings like us do and believe by the reasons that lead them to choose to do it or to believe it. For example, actions can be explained by the beliefs and desires that are responsible for them, and beliefs can often be explained by the perceptions and established beliefs that are responsible for them. When we are explaining the actions or beliefs of other subjects, what is explained are ultimately objects of perception, just as in natural science, for we know about their intentions and beliefs of others only by perceiving their behavior. Some of that behavior is, of course, verbal behavior, which is especially revealing, but this kind of explanation can also be given of other animals, notably, mammals. What makes human beings basically different is that they are reflective subjects. That is, in them, beliefs, desires and perceptions are not mere causes of actions and belief, but causes that have effects on other beliefs or behavior by way of the subjectís reflecting on them. These causes are so special that they are called "reasons." Furthermore, what enables us to identify these causes and see their roles in causing action and belief is reflection.
Reflection plays a role in rational-cause explanation that is analogous to the role of spatial imagination in ordinary efficient-cause explanations and the laws of nature cited in more formal scientific efficient-cause explanations. What enables us to connect cause with effect in the case of rational explanations is reflection on our own capacity for reasoning. When we explain another personís action by citing certain beliefs and desires, our ability to tell the relevance of those beliefs and desires as causes of the action in question comes from reflecting on what we would do if we had certain desires and we believed that we were in the relevant situation. Likewise in seeing the relevance of reasons as causes explaining certain beliefs, we reconstruct the argument in our own brains.
Rational explanation works well enough in the case of the actions and beliefs that occur in the ordinary practice of carrying out our lives. Insofar as the actions and beliefs to be explained have to do with moving bodies around in a world of objects in space in order to satisfy desires, we can understand the causes of the otherís behavior by reflecting on what our own spatial imagination would lead us to do in the situation. That is the kind of behavior that can be explained rationally in other animals. But we can usually reach agreement about ordinary social interactions of human beings as well, because members of a society share expectations about one anotherís actions and beliefs. To explain a particular action or belief is usually just a matter of identifying which of the familiar reasons happened to be responsible for it in that case.
Agreement about which is the best rational explanation is reached easily in such ordinary causes, and it can be seen as an application of the empirical method. Familiar reasons are the simplest in the sense that they fit into the background of beliefs and desires that people share, and we usually prefer explanations that require the fewest familiar reasons to explain any particular action or belief. In short, we assimilate their behavior to what is normally expected. Furthermore, the scope of such explanations is maximally large, because the rational explanation is confirmed by how normal expectations also explain other aspects of the personís behavior.
Actions or beliefs that are unusual, however, cannot be assimilated to the normal pattern. They call for rational explanation in a way that can also be seen as an application of the empirical method. We start, as always, from the neutral background of ordinary behavior and beliefs with generally accepted reasons in the society and we try to identify the special reasons that are responsible for the unusual beliefs or behavior. These are desires, beliefs or perceptions that stand out as different from that neutral background, and since the empirical method requires us to explain the most with the least, we look for the explanation that requires the fewest deviations from the background and the simplest (or most plausible) ways in which they might deviate. And we look for the combination of such deviations with the largest scope. The same beliefs and desires can cause many different actions and beliefs, and thus, we prefer the rational explanation of the action (or belief) in question that can also explain other actions (or beliefs). The more of a personís behavior that a rational explanation can explain, the better the explanation, other things being equal.
Though each of us may use the empirical method to decide what to believe about the reasons for a personís behavior or beliefs, this may not lead us to agree on which the explanation. The problem is that rational explanation depends on reflection, rather than just perception. Each of us must use our own processes of reasoning to judge which possible reasons explain the most with the least. Those reasoning processes involve our own beliefs about the world, the perceptions that we have had, our own desires, values and what we have already decided to do or believe on the basis of them. And the further what is being explained is from the familiar, everyday actions and beliefs that we have all made part of our way of viewing the world, the more differences tend to show up in how we think. People have vastly different views about the most general and basic issues, such as the nature of the world, what is possible, where beings like us come from, what is the purpose of life, what is good and bad, what to strive for, what is worth worshiping, and the like. And such differences extend into everyday actions and beliefs when those giving the explanations come from different cultures. Since what is the best rational explanation depends, in part, on which set of background beliefs and goals the explainers themselves accept, the empirical method does not, in general, make it possible to reach agreement.
It is widely recognized that the social sciences and humanities are not as objective as the natural sciences. But that is not an indication of any inherent weakness in the empirical method. It is, rather, an indication of the difference between the forms of understanding that are required for the explanations involved. Spatial imagination is more uniform than rational imagination, and that makes it easier for people to agree about which theory explains more with less. What the relativism of the social sciences and humanities shows is not the weakness of the empirical method, but the weakness of rational explanation (at least, as long as we come from different cultures and have different basic beliefs and values).
Ontological-cause explanations. The empirical method can also be used in philosophy (and science) by inferring to the best ontological-cause explanation of the world. The nature of ontological explanation has already been explained: it explains the existence of everything found in the world by showing how it is constituted by basic substances and the basic relationship by which they exist together as a world.
This kind of explanation is intelligible to us because of our spatial imagination (that is, the capacity to think coherently about spatial relation and how they change as a result of motion). That is the same capacity on which efficient-cause explanation depends. The difference is that what is being explained by ontological explanations includes the existence and basic traits of the objects found in the world, such the fact that objects have spatial relations and that change is possible, not just what happens to them. But an adequate ontology must also be able to explain why (true) efficient-cause explanations are true. The relationship between an efficient cause and its effect is a kind of regular change, and an ontology must show how the regularities described by the basic laws of physics can be just aspects of basic substances enduring through time with the basic relationship that makes them parts of the same world. That is how ontological-cause explanations are more basic than efficient-cause explanations -- they explain the premises of efficient-cause explanations, both the laws of nature and the initial and/or boundary conditions. .
Ontological explanations differ from one another in the kinds of basic substances they postulate and what they assume about how substance exist together as a world, and empirical ontology decides which is true by which offers the best ontological explanation of the world, that is, which explains the most with the least.
Scope. It might seem that ontological theories are all alike in scope, because they all claim to explain the possibility of everything found in the world. The failure to account for any aspect might be said to show that it is not an ontological explanation at all, must less an adequate one. This is not quite true, however, for two reasons.
First, because there is a difference between explaining and merely assuming. The causes by which an ontology explains the world are the substances it postulates and the basic relationship it takes them to have, and thus, to the extent that what is being explained about the world is the same as what is assumed by the ontology, it is not really explained, but merely assumed. To some extent, that may be true of every possible ontology, but the best one will be, other things being equal, the one in which more is explained and less is merely assumed. That one has the greater scope.
The second reason is that, in an ontological explanation, there is a difference between explaining the possibilities of aspects of the world and explaining their necessity, and the more aspects of the world that are shown to be necessary, the better the ontological explanation.
What an ontology entails about the world holds necessarily. Though that determines the range of what is possible, contingent aspects of the world are left to be known though experience of what is actual. An ontology does not itself explain why certain contingent conditions are actual and others not; that requires an efficient-cause explanations. However, since it must explain the possibility of what is contingent, it may be said to "account for" whatever falls within the range of the possible.
Thus, the minimum requirement of an ontological explanation is that it, at least, "accounts for" everything in the world (in the sense of showing that it is possible). And if anything is found in the world that could not exist, if the ontology were true, then the ontology must be false. But ontologies that are not falsified may differ in the range of what they show to be necessary and what they imply is merely contingent. The principle of explaining the most by the least would require those committed to the empirical method to prefer ontological explanations in which more about the world is shown to be necessary and less turns out to be merely contingent. Thus, there is another possible difference in scope among ontological theories
Simplicity. The simplicity criterion requires us to prefer the explanation with the simplest and fewest causes, other things being equal. In the case of ontological causes, the explanation with the simplest and fewest causes would be the one that postulates the simplest and fewest kinds of basic substances and simplest basic relationship among them. Thus, given two ontological explanations with the same scope, the empirical method requires us to prefer the one that postulates the simpler basic substances, the fewer kinds of basic substances, and the simpler basic relationship among them.
Though it is generally clear which theory has the fewer basic substances, it may not be clear which kinds of basic substances and which basic relationships are simpler. From what we have assumed about the essential natures of basic substances and relationships, however, there is one clear criterion. We have seen that the essential natures of substances may be temporally simple or temporally complex, depending on whether their essential properties exist fully at each moment or they are dispositional and have to do with regularities about how contingent properties change over time. And we have seen that there are also such differences in the simplicity of the basic relationship by which an ontology describes how they are parts of the same world. Thus, given two ontological explanations with the same scope and same number of kinds of basic substances, the empirical method requires us to prefer the ontological explanation whose substances have the simplest essential natures and the simplest basic relationship to one another.
When all these criteria weigh in for the same alternative, the empirical method is decisive. But trade-offs among them can keep the empirical method from telling us which ontological theory to believe. That does not necessarily mean, however, that limitations in the mechanical application of these criteria can be used to argue that no choice can be made among theories in which there are trade-offs. It may still be obvious, when specific trade-offs are considered, which one explains the most with the least.
The rational method. For epistemological philosophy, by contrast, the method of choosing what to believe is not the empirical method, but the rational method. This is not quite the same as an inference to the best rational-causal explanation, because what epistemological philosophy needs in order to be a kind of philosophy is a foundation from which to prove necessary truths about the world. What makes epistemological philosophy different from ontological philosophy is that it uses as its foundation a theory about the nature of reason rather than a theory about the nature of the substances constituting the world. And the necessity of its implications comes down to their certainty, given the certainty of the epistemological foundation. Its reliance on a theory about how we know about the world is what earns it the name "epistemological" philosophy (epistemology being, literally, the explanation of knowing). Moreover, such a foundation is secured by reflecting on how we know. As we have seen, reflection is what enables us to give rational-case explanations of the beliefs and behavior of other beings like use. But epistemological philosophy uses reflection to explain how reason works in general. That is, it uses reason's own power to reflect on how it works to defend a theory about how reason works, rather than merely to say which reasons are responsible for particular conclusions about what to believe or do.
Its theory of how we know is supposed to show that certain truths must hold of the world, and its success in using its foundation to prove necessary truths about the world is called realism. Since it would show that something exists beyond its epistemological foundation, it typically leads to metaphysical dualism of one kind or another (as we have seen in Ontology: As realism).
The theories about the nature of reason used by epistemological philosophy are all based in one way of another on a faculty of intuition, which is taken for granted. (The reason for this reliance on intuition is explained in Change: Evolutionary stage 10: The career of epistemological philosophy.)
There is, however, so little agreement in the history of philosophy about the nature of reason that the best way to explain the rational method of epistemological philosophy is to survey the main kinds of theories about the nature of reason that have developed in the history of philosophy.
Ancient Philosophy. Plato assumed that we know by a kind of intuition in which the objects of knowledge are present to the subject. In the case of perception, they are visible objects in space which can move and interact with other objects, and these he assumed were parts of what he called the "realm of Becoming." We also have a capacity to reason about things, in which we understand their natures, and the objects that are present to us in this way of knowing are what Plato called the Forms, which he believed exist in a realm of Being outside space and time. His "doctrine of recollection" is a myth that explained this rational intuition as resulting from our immortal souls having existed in the presence of the Forms prior to our acquiring bodies in the realm of visible objects. Since the objects of rational intuition are the natures that we recognize in visible objects, he thought that the Forms were responsible for visible objects having whatever natures they seemed to have. Thus, by intuiting the Forms directly, we could know truths about them that are necessary relative to perception, that is, our ordinary way of knowing. That included knowing what is good about visible objects, since the Forms were supposed to follow from The Good Itself and visible objects were supposed to be striving to be like their Forms.
Aristotle also understood perception and reason as forms of intuition that make their objects present to us, though he explained them differently. Perception was supposed to be the result of our sensitive soul taking on the same kinds of sensible forms that exist in the particular substances, and reason was supposed to be the result of our rational souls taking on the essential forms of the objects as a result of "induction" from our perceptual experience of many instances of their kinds. Knowing the essential form of an object gives us knowledge of what holds necessarily, because according to Aristotle, there are final causes at work in nature that make natural substances change in the direction of an end state which is the fullest actualization of their essential form. Not only does that explain certain changes that they undergo, but it also tells us what is good for them. This knowledge, Aristotle argued, was prior to the received, ordinary ways of knowing the true and the good.
Medieval philosophy. Medieval philosophy is basically a continuation of Platonic dualism, except that The Good Itself, or a Form, is replaced by God, or a person. Thus, it retains the theory about the nature of reason on which ancient epistemological philosophy was based. If anything was new in the Medieval period, it was how the new view about the nature of the transcendent being was used to argue for its existence. And the main reason that these argument for the existence of God were not compelling in the end is that they are based on the assumption that principles recognized to be valid within the natural world can be applied to the natural world as a whole.
The belief that every event has an efficient cause can be used, for example, to show that there must be a first cause, when it is assumed that the world as a whole is an event to be explained. Final causation affords a similar proof of the existence of God. Given that every natural change within space and time has a final cause, it could be argued that there must be a final cause of the natural world as a whole, as long as it was assumed that the world as a whole is a kind of natural change and can be explained by the same principle. The argument from design works in the same way. Given that artifacts can change for the sake of an end that is good for them only because they are designed to do so by their creator, the fact that nature itself involves change for the sake of ends that are good could be used to show that there is a creator who designed the natural world to bring about such ends. Even the argument from the recognition of a difference between better and worse to the existence of something that is best can be used to show the existence of God when it is assumed that the world as a whole is not the best.
The ontological argument was the most original use of the rational method in the medieval period to prove the existence of God, and given our assumptions about the nature of existence, we can see the fallacy involved in it. As Anselm put it, since we can think of being "than which none greater can be conceived," God exists. For if the being we are thinking of did not exist, there would be a greater being, namely, one with all the same perfections we were thinking of plus existing. The premise of this argument is that absolute perfection entails existence. But if existence and essence are the two basic aspects of the nature of substance as substance, existence is not entailed by perfection, for perfection characterizes a things essential nature and that is a different aspect of any substance from its existential nature. The perfect being would exist only if he is a substance, and not just a conceivable essence.
This is not quite Kantís critique of the ontological argument, for he argued that existence is not a property at all. On our theory about the nature of substance, existence is a property, albeit a very basic property ó as basic as having an essence is. Having both properties is what makes something a substance.
Modern philosophy. Modern philosophers had a fundamentally different theory about how we know, for they had given up naÔve realism about perception and recognized that the appearance of the natural world in perception is part of the subject, which they understood as ideas in an immaterial mind. That was also to give up the belief that reason is a direct intuition of Forms existing independently of the mind. But on reflection, they found certain ideas in the mind whose truth they could not doubt, and such so-called clear and distinct ideas were taken to be truths that hold necessarily. Descartes believed that clear and distinct ideas enabled him to prove (by way of proving the existence of God) that a natural world exists independently of the mind and is the cause of our perceptions. He also believed that this showed that the natural world has the essential nature of extension, and thus, he claimed that philosophy provided knowledge about the natural world that is necessary, relative to what is known by perception. Since rational knowledge is prior to what is known by experience, Descartes believed that he had justified the method of modern science as a way of learning the details of natural mechanisms. Other rationalists, such as Spinoza and Leibniz, argued from similar theories about the nature of reason to necessary truths about the natural world.
Kant defended necessary truths about the natural world on a theory about how we know that sees the mind as constituting in part what is known, including the natural world investigated by science. Thus, Kant could argue that the part of what is known that depends on the mindís contribution is a priori knowledge about the natural world, holding universally and necessarily relative to what perception discloses about what is actual in the world, or what he called synthetic a priori.
Kantís theory of knowledge forced him to deny that we could know the real nature of things in themselves, that is, what really exists independently of mind, but Hegel adapted Kantís theory of knowledge in a way that enabled him to claim for philosophy the power to know the real nature of the world. He assumed that that the object of knowledge was entirely constituted by a mental substance through what he called dialectical reason, and thus, by reflecting on the nature of dialectical reason, Hegel also thought that it was possible to show what holds necessarily about the world, relative to what is known by science or other ordinary ways of knowing.
Contemporary philosophy. Even contemporary analytic philosophy had a rational method of knowing what is necessary about the world. They assumed that as users of language, we know the meanings and reference of the terms and sentences we use. Though we can use language to describe what we observe in the world and, thereby, follow the empirical method in science, they argued that there are certain truths that hold necessarily about the world because they are entailed by the meanings of the terms we use. Thus, analytic philosophy had a rational method for justifying necessary truths, though it was much less ambitious than earlier kinds of epistemology, because what is necessary was limited to analytic truths.
In each era, there have also been skeptics about the rational method, especially when they entailed kinds of ontological dualism, such as form and matter and mind and body, in which it was hard to explain how the two different kinds of substances could be related as parts of the same world. The inability to answer those skeptics led to doubts about the rational method itself and ultimately to the demise of epistemological philosophy.
 It might be argued that ontological philosophy relies on only one assumption, naturalism, because the other two assumptions might be shown to be consequences of it. We are defining naturalism as the assumption that the world is just what exists in space and time. Since that is an ontological definition, we might already be committed to explaining the natural world by substances and the relations among them, for we will need self-subsistent entities of some kind to explain its existence. Thus, naturalists already accept, in effect, the validity of ontological explanation. And since the world of objects in space and time we mean is the one that is disclosed to us by perception, we might already be committed to using what is perceived as evidence in choosing what to believe about it. Thus, naturalists already accept the empirical method, assuming that the standard of the best explanation is implicitly in the nature of the explanation being given. Hence, naturalism might be said to be the sole assumption for the foundation of ontological philosophy. But the argument is not put that way here, because to start by trying to defend a way of knowing about the world (or a way of explaining it) as implicit in naturalism would obscure the difference between ontological and epistemological philosophy. In the present context, it is better simply to distinguish the three assumptions and make them independently, since they are all equally plausible.