The great dividing line in human thought is between those who believe reality is really real and those who hold that what is commonly called reality is not as it seems. Those who adhere to the former can be broadly called realists, followers of what also has been called the “perennial philosophy.” Realists believe that the world exists independent of the human mind and is knowable by the human mind. Realists, while recognizing that sensory knowledge is limited and not infallible, believe that the world is basically as it appears to us. Our sense knowledge is trustworthy; it is accurate to say that the world objectively is as we commonly understand it to be. We see a tree and a rock and know that the tree is different from the rock in fundamental ways that are understandable to us. A realist believes that this is because the tree really is more or less the living thing we take it to be and the rock really is just a rock. Realism is a philosophical defense of common sense.
Realism, though under attack from the very beginning, formed the bedrock of western thought, from ancient Greece until contemporary times. At times and places in this long history realism has been strong or weak, a driving force behind cultural development or a defensive check against cultural decline. Over the past 400 years realism has been mostly on the defensive. Though today it is weaker than at times past, there are indications that many are ready to turn to realism, making it a force in determining culture once again.
The Beginnings of Realism
As far as we know, the quest to know what is real in a scientific way began with the pre-Socratic philosophers in ancient Greece. Their initial attempts at explaining reality assumed that reality is fundamentally reducible to just one thing. Thales, for example, thought the fundamental element behind reality is water. Others focused on a different fundamental element, such as earth, air or fire, or some combination of these elements.
For Heraclitus, who mentions fire as an example, all reality can be reduced to change. He asserted that the only constant is change, that all reality is a state of becoming, with no permanent natures or essences. Thus it is impossible to place one’s foot in the same river twice. In the time it takes to place one’s foot in a river and remove it the river has so changed that it is not the same river. All reality is this way, a continual state of becoming such that nothing remains the same from one instant to the next.
Parmenides took an opposite position, asserting that change is an illusion, that what exists is unchanging being, known by reflection rather than by untrustworthy senses. Since being simply is and non-being is not, it is impossible for being to change. The only thing that could cause being, understood this way, to change would be something other than itself. But the only thing other than being is non-being, which does not exist. So it seems that change is impossible since there is literally nothing that can cause it to come about. Likewise, motion is not real since any movement requires that one first go half the distance and then half of the remaining distance still needs to be crossed and so on forever, the end never being reached.
It is significant that these and other pre-Socratic thinkers sought to understand reality in a fundamental way. The quest to know about reality has been going on ever since, a fact that attests to the longing of people of all times for such knowledge. It is natural for us humans to ask questions about the fundamental nature of reality and ponder the answers to such questions. Metaphysics is the science that does this in a rigorous way, seeking scientific knowledge about the ultimate cause and the first and most universal principles of reality. It is the science that studies being – that which exists – as being.
Even so, the pre-Socratic attempts at metaphysics all fell short by assuming that reality is fundamentally reducible to just one thing. It seems that when faced with a complex reality there is a human tendency, perhaps based on wanting to have knowing comprehension or even control, to break the complex thing down to one simple thing. For Heraclitus this one thing was change and for Parmenides it was undifferentiated being. These two early attempts at metaphysical knowledge are examples of the philosophical error known as reductionism, an error that is very much present today. Reductionism is the belief that the whole is nothing more than the sum of its component parts. It is present today especially in the assumption that if we understand what a thing is made of and how it is put together, then we know basically all that there is to know about it.
Parmenides was closer to the truth than Heraclitus. Heraclitus’s assertion that change is fundamental results in seeing reality as a process of becoming so radical that human thought and language ultimately break down. One of Heraclitus’s followers, Cratylus, pointed out that it is not possible to use language to communicate since words do not refer to things with enough permanence to be accurately named. In the end, there is not enough stability for something to be considered as a something, rather than merely part of a giant cosmic flux. Heraclitus’s position necessarily leads to relativism, the belief that there are no objective truths. If everything is in a state of flux, we are not able to say anything meaningful about any part of this constantly changing landscape. Heraclitus’s position violates the principle of non-contradiction, which states that a thing cannot both be and not be at the same time and in the same respect. If everything is in a radical state of flux, then what exists lacks the stability necessary for real assertions to be made about it. For Heraclitus, what appears to us as an individual thing, such as a bird or a flower, lacks the unity and stability necessary for us to be able to make a real assertion that could serve as the basis for applying the principle of non-contradiction.
For Parmenides, since reality is nothing other than undifferentiated being we are similarly left without the ability to meaningfully speak about particular things. But Parmenides was correct to recognize that being exists. And when he argued that only non-being could be that which moves being, he was also correct in relying on what has become known as the principle of causality; that whatever is moved – by which realists mean changed in some way and not only moving from one place to another – is moved by something else. Nothing changes itself. His argument likewise assumes that being cannot contradict itself, which is an implicit commitment to the principle of non-contradiction.
Realism as a school of thought began when the early attempts at reducing reality to one thing were set aside in favor of allowing for a composition, a duality that resists reduction. Aristotle, perhaps the greatest of the early realist thinkers, recognized that both Heraclitus and Parmenides were partly right, but that reality is more complex than either thought. But it was not enough for Aristotle to simply recognize his predecessors’ insights and deficiencies; he strove to understand exactly where they fell short. Aristotle saw that individual things are what they are, that they have certain characteristics and perfections. He also saw that they change in various ways; these include changes that do not affect what a thing essentially is, and other changes that result in one thing becoming something else, so that it no longer exists and instead a new thing comes into existence. A field mouse can run across a field, changing its location but not the fact that it is a field mouse. It can also be caught and eaten by a hawk, changing essentially by dying and partly becoming the hawk for which it provides nourishment.
Aristotle agreed with Parmenides that something needs to account for the changes that we observe in the world around us. He disagreed that the only possible candidate for changing being is non-being. Rather, Aristotle recognized that there is a fundamental distinction that needs to be applied to being, the distinction between act and potency. Act is any perfection of a thing; it is related to what a thing actually is. Potency is a real capacity for change that exists in a thing. This distinction applies to all objects of our experience, to everything we see, hear, smell, touch or taste.
Reflecting on how things change can help us better understand act and potency. We see, for example, the dog Fido. There are several ways in which Fido is, aspects of how Fido exists as a particular dog. Fido has a certain size, a certain age, a certain color, identifies with a certain family, in some sense seeks to defend this family by barking at strangers who walk by and loves to chase rabbits and cats. Many other aspects of Fido’s existence could be stated, the sum total of which would be Fido’s actuality, his being in act. But Fido also is able to change in certain ways; he has the potency or potentiality to change, or stated more technically, to become actualized in new ways. There are many ways he can change without ceasing to be Fido. He may move from one place to another, he may lose some of his hair when groomed, or he may go from a lazy half sleep to a fully aroused frenzy when he smells the neighbor’s cat in his yard. All of these changes are “accidental” changes, since they modify Fido in a non-essential way. Fido is also able to undergo one essential or substantial change, specifically death. He may die of old age or from a sudden trauma like being hit by a car. In either case, when he dies he no longer is the same Fido. He will decay and become part of the soil where he lies.
Aristotle realized that the act-potency distinction is what Parmenides was missing in his denial of change. Parmenides was correct that all change needs a cause, but the cause of a thing undergoing a change need not be non-being (indeed, since non-being is nothing, it is unable to cause anything) but rather a thing that is in act relative to the potency of the thing changed. When a cold pan is heated over a campfire, the potency of the pan to be hot is realized by the actually hot wood fire. Realists will say that change is simply the reduction of potency to act by something else that is in act. When the field mouse is eaten by the hawk, the mouse ceases to be a mouse when it is incorporated into the hawk. It partly becomes the hawk and partly excreted waste. The field mouse, unlike a rock, is potentially food for the hawk. Fido likewise has the potency to change in specific ways, both accidental and substantial.
So the many varied things around us are things that have both act and potency. With Heraclitus, Aristotle agreed that individual things can change, that they have the ability to do so, an ability called potency. With Parmenides Aristotle saw that things have specific natures, that they really are things, that they are actualized to a certain extent. Aristotle embraced the distinction between act and potency. A thing is in act in so far as it is what it is and a thing is in potency in so far as it has a real capacity to change in some way. Being is not simply undifferentiated existence, but rather each thing both exists in a certain way – is actually something – and has the potential to change in some way.
The first of the twenty-four Thomistic Theses summarizes as follows: “Potency and Act so divide being that whatsoever exists either is a Pure Act, or is necessarily composed of Potency and Act, as to its primordial and intrinsic principles.”
 Our knowledge of the ancient world is limited. For example, only about one third of what Aristotle wrote has survived, even though he was famous as the tutor of Alexander the Great.