The career of epistemological philosophy.  Pre-Socratic philosophy was a radical random variation on the arguments of rational level culture, and it may also have been tried out in other civilizations. But only in Western civilization did it give rise to epistemological philosophy and put the linguistic structures generating social institutions on the philosophical level of neurological organization.

Ancient and medieval epistemological philosophy. Epistemological philosophy began when Plato discovered a convincing way of constructing an argument that would explain the validity of all first level arguments based on perception and desire. It was also an explanation of everything, but it was not based on a theory about change and diversity, that is, about efficient causes. Instead, it was a theory about the nature of reason based only reflective understanding, or the capacity of subjects to use rational imagination to explain rational causation. It did entail an explanation of the nature of the substances constituting the world, but that was an afterthought, for its approach to philosophy was epistemological. The theories about the nature of reason and the nature of consciousness that we have derived from our ontological foundation fit together as a way of understanding the basic structure of epistemological arguments. We need only consider what rational subjects had to work with, when they turned to reflective understanding for a theory about how we know about the world, because there are only certain ways that those elements can be used to explain the validity of the reasons used in the ordinary arguments of rational level culture.

What represents the current scene to the subject are the telesensory images that are currently being used (in conjunction with input from the current bodily condition) to construct a local image, and together with the representation of the body itself, that is the subject’s perception of the world. But the local image (and the body image to which it is related) generated from current sensory input are embedded within a faculty of rational imagination, and thus, rational subjects are able to see actual states against the background of what is possible by efficient causation by using covert behavior to call up all sort of images in relation to them. Consider the aspects of the world that are represented in rational imagination.

Spatial imagination makes it possible to call up sequences of images representing the effects of the locomotion (or motion) in relation to other objects within the local scene and to call up sequences of local images (that have been recorded in memory as a map of one’s territory) representing locomotion beyond the local scene. Or spatial imagination can be used more generally to think about the effects locomotion and turning (or motion and change of direction) within the local scene, in relation to a purely imaginary local scene, or in the abstract (because the same behavioral schemata are used relative to different local images).

Structural imagination makes it possible to call up sequences of telesensory images, or object images, representing the effects of manipulating objects in the local scene, such as rotating and twisting them, and it too can be used abstractly to think about the geometrical structures of objects in space.

Naturalistic imagination, which comes with the ability to use natural sentences, makes it possible to call up sequences of images, or naturalistic images, representing the states of affairs that make natural sentences true, so that together with (particular and general) beliefs about regularities (which are either built into the structure of imagination or acquired from experience), rational imagination can represent their effects in the natural world.

Rational imagination, which comes with the ability to use psychological sentences, makes it possible to call up sequences of images as psychological predicates (or psychological images) and to predicate them of objects in space that are subjects, and this ability to think about psychological states is the ability to understand how they are causes or effects of other psychological states, including their role as reasons (or causes that are represented as causes as part of the process of causing beliefs or behavior). 

Perception and rational imagination both have an appearance to the subject. These images are certain configurations of neurons firing in various 2-D arrays of neurons that are connected by projections between the thalamus and neocortex (or between regions of neocortex), and their firings are all synchronized by the thalamus insofar as they have to do with the same objects. That is, the brain processes the information contained as patterns of firing in 2-D arrays of neurons, and thus, what happens in the brain is a highly structured in both space and time. But what is more, the joint firings of those neurons is like a complex antenna that generates a steam of photons, and the intrinsic natures of the photons being given off by the active brain are phenomenal properties which make the rational subject conscious.

Perception and rational imagination have different appearances, because the images they involve are caused in basically different ways. Perceptual images come from sensory input (and involve projections from the sensory organs through the thalamus to the neocortex), whereas images of rational imagination come from covert behavior operating on memory (and involve only regions of neocortex beyond those registering sensory input). This makes these two kinds of images appear quite different from the point of view of the subject reflecting on them. But in both cases, objects seem to be present to the subject, in one case, as objects of perception, and in the other case, as objects of reflection.

The only plausible way to construct an epistemological argument is to assume that the appearances of these objects in consciousness involve an intuition of objects that exist independently of the subject, for there is nothing else to reflect on, except the feelings or emotions associated with desires (that is, the goal selection system).

The images of perception naturally appear to be objects in space, because the local image represents the objects as having locations in space relative to one’s body. Though the telesensory images are certain groups of neurons firing in 2-D arrays that are located in certain regions of the thalamus and neocortex, they seem to be located in space, because as we have seen, they are combined with other telesensory and somatosensory images as part of a local image, and with spatial imagination, the subject is able to think about the effects of motion relative to them by calling up sequences of images in imagination. The sensory images of objects are seen, therefore, against the background of what is possible by motion, and since that is how the subject understands the structure of space, the objects appear to be located in space. And it is a qualitatively rich appearance, because in conscious subjects, what is happening throughout the brain is registered in the structures of the photons being generated by it.

Naïve (or direct) realism about perception, as this way of interpreting perceptual images is called, is the natural attitude, because there is ordinarily no reason to recognize the difference between perceptual representations in the brain and the objects in space they represent outside the brain. The overt behavior of one’s body actually changes the perceptual images in just the ways one expects. Thus, it is natural to think of perception as an immediate intuition of objects in space, including one’s own body, as if the objects themselves were immediately present to the subject.

Objects of reflection, such as the object images representing objects of various kinds that evolve as the meanings of some general terms, also have an appearance, albeit one that is less vivid, detailed, and persistent, because they are images in rational imagination. They might also ordinarily be taken as objects present to mind by way of intuition, but they do not act like objects in space. They may be imagined as located in space relative to objects that are perceived, but unlike the latter, what changes them is not the overt behavior of one’s body, but the covert behavior by which one calls up images from memory. Imagined object are easier to handle. Not only are they not constrained like objects in space, but neither do they appear to be in time. Though object images involve sequences of images in imagination, such sequences are simply the meanings of the general terms. The meaning of “cube” or “tree,” for example, may include a sequence of images representing the effects of rotating it or moving around to see it from the other side, but that is understood to be just a way of thinking about the nature of the cube or the nature of the tree. The object itself is unchanging and, thus, not in time. The natural attitude is, therefore, to assume that the objects of reflection are in the mind, that is, merely subjective.

However, since objects of reflection are appearances quite on a par with objects of perception, it is possible to think of both as intuitions of objects that somehow exist independently of the subject. Thus, just as the tree that is perceived is seen a located outside the subject in space, so the image of reflection that is the meaning of “tree” can be seen as located outside the body in some other way. The connection between these independently existing objects affords an explanation of the objects of perception, for it is possible that the objects of reflection are also somehow what causes the objects of perception to have the natures they seem to have. That is what Plato did by positing the existence of Forms in the realm of Being beyond the visible objects in the realm of Becoming.

This theory about the nature of reason overcomes, therefore, the dichotomy between the objects of naturalistic and reflective understanding. It uses it as an explanation of what exists in the world. But that was not, of course, the only cause of Plato’s metaphysics. Plato was looking for a metaphysics that would also explain the nature of goodness, that is, a way of overcoming the dichotomy between the true and the good and the dichotomy between the good of satisfying animal desire and a higher good (self interest and spiritual interest).

The dichotomies that philosophy must overcome include not only the difference between the science of nature and the science of subjects, but also the difference between the true and the good. In addition to cognizing the true, reason has the power to guide behavior, and thus, it also seeks to know what is good. Practical arguments became the focus of attention after the Persian wars, when Athens was the dominant city-state and the exchange of arguments was supported by the hiring of teachers, called sophists, to train the sons of the wealthy to be leaders in the promising, new age of independence.

The sophists were itinerants, traveling from one city state to another, gathering knowledge as well as teaching, and this cosmopolitan experience led them to conclude that the standards of justice and other virtues are conventional, that is, true merely because they are believed to be true in a society. In this context, Socrates was on the side of traditional religion, holding that the good is objective, or something about the world that could be known like natural science. But instead of the dogmatism of traditional religion, Socrates insisted that knowledge of the good must be a kind of knowledge that makes the knower virtuous, so that a rational being does the good because it is good and he understands why it is good. That is the meaning of the Socratic principle, that knowledge is virtue. There must be an understanding of the nature of goodness that is so deep that it explains to rational beings why they ought to pursue it. The Socratic principle posed very sharply the problem that philosophy must solve in explaining the relationship of the good and the true. For how can any mere fact about the world show that something is good in a way that gives rational beings a sufficient motive to do it?

Socrates was implicitly asking for a philosophical argument, because he wanted to know what makes all good thing good, which would explain why ordinary arguments about what is good are valid (when they are). And it was his attempt to answer Socrates’ challenge that led Plato to discover the epistemological approach to explaining all the arguments of rational culture. Recognizing that it is possible to think of certain objects of reflection as objects existing independently of the subject in much the same way as objects of perception, as explained above, Plato argued that what makes visible things good is that they are participating or imitating Forms in the realm of Being. This meant that he had to hold that the Forms in the realm of Being are themselves good, and so he argued that all the other Forms follow from the Form of the Good. This was not very satisfying explanation of the nature of goodness, but the transcendence of the realm of Being, or its existence outside space and time, made it possible to think that Being could somehow be the source of goodness. Thus, his metaphysics of Being and Becoming could be used to justify arguments about what is good in a fundamentally different way from ordinary arguments of rational culture, and it was the same way in which he could justify arguments about what is true in the natural world. In both cases, it had to do with visible objects imitating the Forms.

That Plato’s goal was to construct a new kind of argument that would explain the validity of the arguments of rational culture is also evident in his use of this metaphysics to overcome the third dichotomy, between individual and spiritual interest. He argued in The Republic that the state is the individual “writ large.” He showed that the soul of the individual rational subject has three parts: reason, appetites, and a “spirited element” which enabled reason to take control of the body away from the appetites (or what we have found to be the desire to submit to reason). He showed that the functions of these three elements also had to be served in the state by three classes of citizens: rulers, ordinary producers, and an army/police force to enforce the rule of the leaders. He suggested that both are good for the same reason, because of the harmony among the three parts required by their Forms. In both cases, it meant that reason, with the aid of an animal-like power (the spirited element), would prevail over mere animal desire. Thus, Plato defended a view which subordinates the individual to the good of the spiritual animal as a whole in a way that seems almost totalitarian from the contemporary perspective.

The subsequent developments of epistemological philosophy during the ancient and medieval period are a story about attempts to solve problems it caused and how its marriage with Christianity eventually made philosophy the foundation of subsequent Western culture. Only the highlights need be mentioned here, for our goal is merely to sketch the career of epistemological philosophy in order to show how its various forms are variations on the same theme. 

As even Plato recognized, the gulf between Being and Becoming is a major problem with his metaphysics. How is it possible for such opposite kinds of entities as unchanging objects of rational intuition and changing objects of perception to be related as parts of the same world? Plato found himself holding (even in the Timaeus, where Becoming is explained as being constituted by the “receptacle,” or space, and “moving images” of the Forms) that they are different substances, and in order to defend his epistemological argument for the independent existence of the Forms, it was necessary to explain how these two substances are related to one another. It was Aristotle who attempted to solve that problem.

Aristotle accepted Plato’s epistemological approach to philosophy and posited objects of rational intuition as fundamentally different from the objects of perception. However, he insisted that they were not different substances, but merely irreducibly different aspects of the same substances: essential forms and matter. This afforded Aristotle a more convincing explanation of the natural world, because he could insist that just as the material aspect of particular substances makes them able to act on one another and, thereby, account for efficient causes, so the formal aspect of particular substances makes them subject to final causation, that is, the tendency of essential forms that are merely potential to become actual, and thereby account for functional explanations. This teleological view of nature enabled Aristotle to account for the regular changes observed in biological organisms, and he extended the same kind of explanation to physics and astronomy.

Teleology gave Aristotle a theory about how the good is related to the true that resembled Plato’s, because he could hold that what is good for any substance is the full actualization of its essential form. The difference is that, having denied the existence of a realm of Being, Aristotle could not hold that the essential forms are explained by the nature of goodness (The Good Itself, as Plato called it). He had to argue that the good is different for different substances (and, thus, that the only reason it is good is that its essential form happens to exist in the world). Aristotle attempted to explain the relationship between individual interest and spiritual interest by holding that rational animals are essentially social (though he did not explain how substances with one essential form could jointly constitute a higher level organism with its own essential form without giving up their essential form as individuals).

The difference between essential forms and matter confronted Aristotle, however, with the same kind of problem that faced Plato, for as he recognized, there had to be an explanation of the relationship between them. This led Aristotle to argue in the Metaphysics that individual substances are basically essential forms and that the material cause is merely their particular existence, or as it came to be called, a mere “principle of individuation.” (In terms of the nature of substance as explained here, Aristotle tried to avoid holding that form and matter are basically different substances by reducing the difference between form and matter to the difference between the essential and the existential aspect of each particular substance.)

Though Aristotle tried to naturalize Plato’s metaphysics by denying the existence of a separate realm of Being, Plato and Aristotle were both realists about forms. Both believed that, in addition to perceptible objects, there are intelligible objects. But since what they were talking about were actually images in the faculty of perception and (certain) images in the faculty of rational imagination, which have a phenomenal appearance to the subject, it is not surprising that there is no adequate explanation of the relationship between them when they are taken to be objects existing independently of the subject, regardless whether it is conceived as a relationship between visible objects and Forms or between matter and essential forms. The inability of realists about forms to formulate a metaphysics that could explain adequately how they are parts of the same world as material objects in space led to doubts about their existence, and thus, realism gave rise to anti-realism. Anti-realism was acted out mainly during the Roman era.

Philosophy continued to be discussed by educated people in the Roman empire, but the two most popular philosophical systems abandoned realism about forms in favor of materialism. The Epicureans believed in atoms and the void, and the Stoics believed that the world is constituted by two kinds of matter, ultimately, active matter and passive matter. (Active matter replaced essential forms as the cause of the order found in nature, for it was supposed to give passive matter into all the proper structures and behavior.)

Neither was much concerned about overcoming the dichotomy between naturalistic and reflective understanding, for both simply took it for granted that rational subjects are part of nature. But they called themselves philosophers, because they were interested in overcoming the dichotomy between the true and the good. They prized Greek philosophy as the model for the higher form of reasoning that would give them wisdom, though the kind of wisdom they sought was practical. Epicureans followed Democritus in defending hedonism, the view that pleasure is the one and only ultimate good and pain the only ultimate evil. They used the determinism of atomism to argue that rational beings cannot help but pursue pleasure and avoid pain. Stoics held that the good life is to suppress all desire for anything different from what happens. The believed that everything happens for the best, because active matter pushed passive matter around in a way that makes the world as a whole a perfect being.

Even as an attempt to overcome the dichotomy between the true and the good, however, Epicureanism and Stoicism were less philosophical arguments than the attempt to have an alternative to traditional religions in thinking about how to live. Neither even attempted to explain how the true makes the good good except to insist that the highest wisdom of philosophy is to make peace with natural necessity. Epicureans never tried to explain why there ought to be rational beings in the world who must pursue pleasure, and the Stoics never explained what it is that makes the world shaped by active matter perfect.

Medieval epistemological philosophy. Toward the end of the Roman era, there was a revival of interest in Greek philosophy as a way of overcoming the dichotomy between naturalistic and reflective understanding. (Plotinus formulated a variation on Plato’s metaphysics that tried to overcome the dualism of Being and Becoming by taking the ultimate source of everything to be the One and explaining the rest of the world as levels of emanations from it.) But Plato’s dualism is what sealed the marriage of Greek philosophy with Christianity, giving Western civilization a uniquely philosophical religion. Later, with the inclusion of Aristotelian philosophy, its rationalism was complete, and the effect on subsequent civilization was profound.

The Judeo-Christian belief in a God who created the natural world combined easily with Plato’s metaphysical dualism of Being and Becoming. Being could be reinterpreted as a supreme rational being, that is, a person. (Plotinus had already portrayed the Forms as aspects of a self-thinking being in the first emanation from the One.) Since God created the natural world, it was possible to take God to be the objective source of goodness that Socrates and Plato were seeking. Thus, Plato’s way of overcoming the dichotomy between reason and nature was resurrected.

But Augustine was the matchmaker, and his belief that it was simply the will of God that made the good good undercut the rationalistic intent of Socrates and Plato by implying that it is arbitrary. However, with the rediscovery of Aristotle in the Middle Ages, that defect was corrected by Aquinas. He argued that what God knows, rather than his will, explains why the good is good. That is, God’s self-understanding includes an explanation of the nature of goodness that reveals why the good ought to exist. And since that knowledge of the nature of goodness is what guided God to create a world like ours, His will was free. God turns out to have the wisdom that Socrates was seeking.

The upshot is that the belief in wisdom as a higher form of argument that can give us a seamless and complete understanding of the true, the good, and the beautiful became, though its adoption by Christianity, a basic principle in the evolution of the arguments about social roles that generated the institutions of Western civilization as the Middle Ages gave way to the Renaissance and the modern era began. The belief that social roles had to be justified by basic principles about the nature of morality and justice that could be known by reason, and the belief that each rational subject has a free will which makes him ultimately responsible for his behavior (and the eternal fate of their souls) led to institutions that recognized the autonomy of individuals and the sanctity of contracts. That gave the edge to institutions of private property and market exchange that would make it possible for capitalism to evolve, helping to pave the way for ontological philosophy. 

Modern epistemological philosophy. In the modern era, epistemological philosophy took a fundamentally different form, though its theories of reason were based on the same two elements: perceptual and rational intuition. The difference was caused by modern science, another offspring of ancient and medieval epistemological philosophy which forced the recognition that the ancient atomists had been right to reject naïve realism about perception.

By the renaissance, mathematical arguments had evolved far enough for it to be recognized that there are quantitatively precise regularities about what happens in the physical world and that they can be represented mathematically. Ever since Plato (or even Pythagoras), mathematical knowledge had been the model for the deeper kind of knowledge about the world that epistemological philosophy was supposed to make possible, and mathematical knowledge evolved as philosophers become mathematicians exchanged mathematical arguments.

On the other hand, the belief that the natural world had been created by God, a rational being, made it plausible to assume that nature had been designed using mathematical concepts. Mathematics was the “language of nature,” as Galileo put it, and thus, it was plausible to assume that the use of mathematics in physics would enable rational subjects to see into the mind of God.

The first advances in physics were all discoveries of quantitative laws of nature, including Kepler’s laws, Galileo’s laws, and before long, Newton’s laws. Even Copernicus had defended his revolutionary view of the universe as a mere mathematical possibility. Mathematics provided the tool that eventually pried open the lid that had long kept reason from understanding micro level processes, leading eventually to chemistry, biology and neurophysiology. Since it was a gift of the previous era of philosophical culture, it is ironic that its first main effect was to replace naïve (or direct) realism about perception with critical (or representative) realism.

The belief that the physical world is made up of substances whose ways of moving and interacting can be described by quantitatively precise laws of nature was recognized as materialism, but it was a form of materialism that had to deny that matter has any of the qualitative properties it seems to have. Those qualitative properties had to be explained as effects on the subject that are caused by the objects through chains of causation that could be explained by laws of nature, which is basically the conclusion to which ancient atomists, like Democritus, had been driven as the conclusion of Pre-Socratic philosophy two thousand years earlier, and for much the same reason. (The belief that shape and size were the only essential properties of atoms was also a quantitative view of matter.) Modern scientists understood that perception of objects in space, for example, by vision, had to be caused in some way by something that travels from the object across space over time to the subject. And since anatomy had made it clear by then that the brain was responsible for receiving sensory input and guiding behavior, there was, within the body, a second leg of the chain of causes and effects that were responsible for how it appears to the subject (implying thereby that the body also lacked the qualitative properties that seemed to be located in it, such as the feel of hot and cold).

This view had a profound significance for anyone who would attempt to take the epistemological approach to showing how the validity of all the arguments of rational culture can be shown by a theory about the nature of reason that was based on reflective understanding. Ever since Plato, epistemological philosophy had been founded on naïve realism, the assumption that the perceptual appearance of the world is an intuition of objects that exist independently of the subject (or else are properties of the same kind as those that exist independently, as Aristotle held). But in the modern period, it was recognized that the appearances of object in perception have a basically different nature from what actually exists independently of the subject. It is called “critical realism,” because it reject the naïve view, or “representative realism,” in contrast to the :direct realism” of ancient and medieval philosophy. Since the perceptual appearances must someone be part of the subject, the subject himself must be a basically different kind of entity from the objects in space. It was called the “mind,” and the appearances of objects in perception were called “ideas of perception.”

Since the implications of this line of reasoning are not well recognized, it is worth emphasizing something about them that confirms our explanation of how the brain works. It is not only the qualitative properties of the objects of perception that are in the mind, but also the appearance that they have locations in space. That is, ideas of perception include the perception of space itself, not just objects in it. Consider, for example, the distance between your face and what you are reading right now. That is a part of space that seems to be as immediately present as the material object on which these marks are inscribed. That is, of course, what we would expect, since the qualitative properties, or sensory qualia, are parts of the telesensory images that are combined along with input about the condition of the body in constructing a local image to represent the local scene. The perception of the distance between your face and the material object embodying the written words is part of the understanding one has of space because of how one can imagine it changing as a result of certain ways of behaving, such as moving your head, turning the object around or moving your body around in the local scene, which is itself seen as just part of an entire world of objects in space. The upshot of this is that what is contained “in mind” is not just sensory qualia, but also a phenomenal space in which all those qualia are located. What one naively takes to be the whole natural word, in other words, is contained in the mind, and what exists independently of it has an entirely different nature, even if it is also assumed to be made up of objects in space. The physical world is made up of material objects in real space.

The modern philosopher who took up the tradition of philosophy and applied it in the modern era was Descartes, and the form of his epistemological argument can also be derived from this ontological explanation of the nature of reason and consciousness. With only perception and rational intuition to use, Descartes used the latter to argue for the existence of the objects represented by the former.

Descartes recognized that the ideas of perception are located in the mind, distinct from objects existing independently as an external world. (That was the point of his doubts about perception based on its similarity to dreaming and the possibility of the ideas being supplied by an evil demon.) For him, therefore, the way to explain the validity of the first level arguments about the natural world by which science was discovering the laws of nature was to show that a world of the kind discovered by empirical science actually exists. That is how he would overcome the dichotomy between naturalistic and reflective understanding. But since his higher level of forensic organization was based on reflective understanding, the only other resources that Descartes could use as a deeper “cause” were other objects of reflection. The ideas of memory and imagination were of little use, since they obviously came from ideas of perception. But there were other ideas, which he called “clear and distinct ideas,” which are certain principles that derive from the structure of the faculty of rational imagination. They differed from perceptual ideas in the same way that Plato’s Forms differed from visible objects, and the prime examples of such ideas were, once again, those of mathematics. But since Descartes was a critical realist, he recognized that clear and distinct ideas are as much part of the mind and the ideas of perception. Plato’s rational intuition of independently existing Forms had become a rational intuition of necessary truths. Thus, in order for this theory of reason to provide a deeper cause explaining the validity of the first level arguments of natural science, he had to argue that clear and distinct ideas could prove that a world of extension exists outside the mind.

His famous argument started with the Cogito, “I think, therefore I am,” his first clear and distinct idea, and proceeded to use other clear and distinct ideas to argue for the existence of a God. God’s perfection precluded His deceiving the finite rational beings He had created, and thus, Descartes concluded that there is a world existing external to mind with the essential nature that rational beings can grasp clearly and distinctly through geometrical reasoning. Thus his theory about the nature of reason explained the validity of the arguments of both reflective and naturalistic understanding, and the proof of the existence of God allowed him to adopt a traditional theological explanation of the nature of goodness.

Descartes’ new way of doing epistemological philosophy was a form of realism, because it took the ideas that are immediately present to mind as its foundation and it tried to prove the existence and nature of a world beyond them. But Descartes’ argument for the existence of the external world was not convincing in the end, and no one has been able to formulate an argument that does what he wanted. Nevertheless, Descartes set the agenda for all of modern philosophy. It would be a battle between realists and anti-realists about the external world. The main obstacle to a proof of the existence of an external world was the fundamentally different natures of mind and body. As Descartes pointed out, body is extended and divisible, whereas mind has a unity that does not admit of such division. That was his argument for holding that God had created them as different substances.

The unity of mind, as we have recognized, is how all the sensory qualia that seem to be located in different places all have an appearance at once to the same subject to which other ideas are also appearing. Consciousness does have a unity that truly does not admit of division like a material object in space.

It may be worth noticing, by the way, that ontological philosophy provides the kind of argument for the existence of the external world that Descartes was seeking. He wanted a clear and distinct idea that would prove the existence and nature of the world external to mind from the point of view of the rational subject, and that is what is provided by this explanation of the wholeness of the world. On the assumption that nothing exists but space and matter (of kinds that explain the truth of the basic laws of physics), not only does it derive reproductive global regularities that explain the essential nature of rational subjects and their place in the world, but it also explains the nature of their consciousness as the intrinsic natures of bits of matter continually given off by active brains. Together, as we have seen, they explains the clear and distinct ideas that Descartes takes to be indubitable. But this explanation is itself a clear and distinct idea in Descartes’ sense. It is distinct in Descartes’ sense (that is, separate from and independent of any idea that is not before the mind), because it is an idea of the whole world, which is everything that exists. And it is clear in Descartes’ sense (that is, with nothing obscure or vague about any of the parts of the idea that is before the mind), because it is an explanation of the entire world and everything in it by the basic substances that constitute its existence. If the rational subject would just look in the right direction, therefore, he would have a clear and distinct idea that entails not only his own existence as a conscious mind, but also the existence and nature of a world that exists independently of mind.

Mind-body dualism was nevertheless an intractable problem in modern philosophy, because it is a form of epistemological philosophy which attempts to explain the validity of ordinary, first level arguments by a theory about the nature of reason that is based on what can be known about reason by reflective understanding. Reflective understanding makes reason seem to be a form of intuition, because all the ideas in the mind seem to be objects of intuition and clear and distinct ideas are just a special kind. But if the subject knows that he has ideas (and, thus, that he exists) because of how they appear, or he knows that clear and distinct ideas are true because of how they appears, the reasons that determine his beliefs can hardly be efficient causes like those that determine what happens in the natural world. Mind must be a fundamentally different kind of substance from body.

The subsequent history of modern philosophy can be predicted, for it is the attempt to vindicate Descartes’ new way of doing epistemological philosophy by overcoming the problems he encountered — or else arguing that it cannot be done, that is, defending anti-realism. In either case, it has to provide some explanation of the validity, if any, of the arguments of rational level cutlure, not only in the science of subjects, but also in the science of nature. At first, it seemed that there must be a way of defending realism about the external world, since mathematics provides an understanding of its essential nature. But the difference in nature between body and mind was even deeper than the difference between Becoming and Being, the two substances of Plato’s metaphysics. Modern philosophers recognized that both realms to which Plato was referring are in the mind (as the ideas of perception and the clear and distinct ideas of rational intuition), and thus, what they meant by the external world was something whose existence Plato did not even recognize.

Continental rationalists like Spinoza and Leibniz hoped to defend realism about the external world by explaining the relationship between mind and body in a different way from Descartes. Spinoza thought mind and body were two different essential natures (“attributes”) of a single substance that constitutes the existence of the entire world, and Leibniz thought that mind and body were both kinds of minds (“monads”) whose relationships, like the monads making up the rest of the world, were a pre-established harmony that God had built into the world from the beginning. But instead of showing how reason could know the existence of an external world, the implausibility of their metaphysical systems brought the whole approach of rationalism into disrepute.

British empiricists, like Locke, Berkeley and Hume, rejected the attempt to use reason to prove the existence of the external world. But they did not give up the Cartesian project. Locke attempted, instead, to explain the validity of the first level arguments of natural science by showing how they are based on ideas of perception alone. But this merely confirmed that the existence of the external world cannot be known in that way, and Berkeley embraced anti-realism about it. Hume agreed, though he focused his anti-realism on causation, showing that perception provides no reason for believing efficient-cause explanations except the regular conjunction of events of those kinds. Though scientists could not share the philosopher’s skepticism about the natural world, they had to agree with empiricists in rejecting rationalist metaphysics, and empiricist skepticism about causation put a real limit on the ambitions of natural science, encouraging natural science to think of its goal as merely discovering the basic laws of nature.

Kant sought to overcome the obstacle that mind-body dualism posed for epistemological philosophy by insisting that the first level arguments of natural science are really about the phenomenal world, that is, the world constituted in part by the mind, not about what exists independently of it. Though Kant did not deny that something does exist independently of mind, he did deny that such “things in themselves” are in space or time. Space and time were mere forms of intuition in the mind. This transformed Cartesian mind-body dualism, because it was no longer possible even to conceive the nature of what exists besides mind. But it did not eliminate metaphysical dualism, because Kant was still a realist about things in themselves outside the mind. And the acknowledgment of a reality that reason could not grasp meant that epistemological philosophy had to admit explicitly that its way of explaining the validity of all the first level arguments of rational culture did not explain the wholeness of the world, but only the wholeness of reason itself. This discovery was more than some defenders of traditional philosophy could accept.

Hegel sought to overcome the obstacle of recognizing the existence of something whose nature reason cannot grasp by constructing from the elements of Kant’s theory of mind a dialectical theory of reason. Instead of helping to constitute a merely phenomenal natural world, as Kant held, Hegel argued that reason constituted the actual natural world and everything about it. By taking individual rational subjects to be merely moments in its dialectic, Hegel could insist that he had shown how reason is able to know the existence and nature of a world existing independently of each particular mind, thereby defending realism, in a sense, and giving a philosophical explanation of why the first level arguments of rational culture (mere “understanding,” in Hegel’s view) are valid. But such absolute idealism merely exposes the real nature of epistemological philosophy as the attempt to discover the deeper cause of the world that is known to rational culture in the nature of reason, rather than in the nature of the world that exists independently of rational beings.

As far as goodness is concerned, the medieval theological explanation was taken more or less for granted during the modern era — until Hume tried to explain what is good in terms of natural desires and Hegel tried to explain the nature of goodness by the perfection of the outcome of his dialectic.