The Legacy of Plato and Aristotle

Philosophy deals with the biggest and most complex issues facing humanity and philosophical insights underpin all human endeavors. Because the questions considered by philosophy are so large and complicated, philosophy itself is a long conversation about those issues among many people across many years. To compare past perspectives with our own today helps us better understand the questions we consider.

That said, because ancient Greek philosophers are so far removed from our time and social milieu, they are less relevant and influential to us than many other philosophers nearer to us in time. We know of several dozen philosophers from ancient Greece and covering them all can easily fill whole books and university courses, but because we have so many philosophers to cover, we will only briefly discuss the two ancient Greeks who stand out as significantly more influential than the others: Plato and Aristotle. These two giants of ancient philosophy were most influential in the conversation about how we understand the objects in the world that we experience.

Understanding the World Around Us

Plato’s most significant influence was his theory of dividing reality and our knowledge of it between two realms of existence. He developed the concepts of particulars and universals. Particulars are individual objects—a tree or a human—and universals are the essential qualities common to all objects within that kind of object denoted by the names—trees and humanity. Plato called these universals, “Forms,” because each particular object partakes of a Form—a tree of the Form of Treeness, a person of the Form of Humanity, and so on.

Plato firmly believed that for knowledge to be genuine, it must be universal and unchanging because truth is universal and unchanging. Particular physical objects change, erode, and eventually disappear, so all truths must be nonphysical. Therefore, genuine knowledge is not perceivable by the physical senses. True knowledge must instead be perceived by reason which can understand unchanging truths. Therefore, Plato sees reason as part of a different realm of reality than the physical realm.

Plato categorizes the different ways that we perceive and think and arranges them in the structure of reality. He drew a symbolic line separating the two realms of existence—the visible realm and the intelligible realm—corresponding to how we can perceive each realm. We can picture Plato’s divided line and the two realms in the following way:

Plato’s Divided Line

At the top is the fundamental principle of reality that Plato calls “the Good.” It is the highest realm of existence, perfect in every way. Plato writes: “The Good may be said to be not only the author of knowledge to all things known, but of their being and essence” (Republic VI.509).

Above the divided line is the intelligible realm and the Forms—the universals such as Treeness and Humanity. They are perfect and unchanging and therefore fully real. This is a nonphysical reality we cannot perceive through our physical senses. Plato refers to it as the “intelligible world” because this realm is intelligible to reason. This highest level of knowledge is the realm of true knowledge through rational contemplation of the universal Forms.

Below Plato’s divided line are the changeable particular objects of the world. Our current bodily existence is in the visible realm of physical objects that we perceive through our physical senses. Our physical senses are so easily deceived that we cannot hope to learn much from observing the visible world. We can, at best, get only partial, temporary ideas about temporary particular objects.

According to Plato, the particulars in the world are mere copies of the universal Forms. Each particular object somehow “participates” in the Form. Plato contends that because objects in the visible realm are mere copies, and change and disappear, they are not fully real and we cannot understand them rationally. Plato theory of knowledge is that to understand reality and what is in it, we should rationally contemplate the universals rather than particular objects.


Aristotle dealt with many of the same questions that Plato asked about how we have knowledge, and also thought of objects in terms of particulars and universals, but Aristotle thought that universals did not exist apart from particular objects. He maintains that universals exist in objects, not separate from objects. The particular objects that we see in the visible realm are what exists. This means that the objects in the world exist as they appear to exist without connection to a hidden, higher realm.

Every object has an essence, the distinct set of qualities that defines the kind of object it is. For example, every tree has leaves, so we identify an object with leaves as a tree, and we understand that possessing leaves is part of the essence of what a tree is. According to Aristotle, the mistake that Plato made was thinking that similarities of kind meant the existence of a universal of that kind. Certainly, all trees share similar qualities that we can understand by the concept “tree,” but to say that there is an essence that we can name as “tree” does not mean that there also exists a Form of Treeness.

In Aristotle’s theory, part of an object’s essence is its potentiality and actuality. Objects all have actuality because we can perceive them; if they didn’t have actuality, then they wouldn’t exist for us to perceive them. But every object also has potentiality—the potential to become something more or something else. All objects that have been actualized into what they are now. They are actualized but also contain the potential to actualize into something else. Only a perfect supreme being with perfect actuality could have no further potential. All the objects we experience are a combination of actuality and potentiality.

Every object has a specific potentiality that determines what that object is capable of becoming. An acorn possesses only the potential to become an oak tree. It does not have the potential to change into a maple tree, cabbage, beaver, or any other object.

Aristotle developed the concept of the four causes to explain how an object’s potentials are actualized. By the term “cause,” Aristotle meant something broader than how we usually use the word. Today we use “cause” strictly in the mechanical sense of a prior event bringing about a result—for example, flipping the switch causes the light to turn on. Aristotle’s thinking is wider. Aristotle uses the term “cause” to describe multiple factors that affect why objects change.

Aristotle identifies four causes—four different reasons that explain objects and change. Those four reasons are the material cause, formal cause, efficient cause, and final cause.

The first cause is the material cause, which is exactly as the name indicates. What is the matter of which an object is composed? The second cause is the formal cause. This also is as the name indicates—the form or essence of an object. The object’s form and matter describe the object as it currently is. An acorn is an object that is a mass of organic plant matter that is in the form of an acorn. An oak tree is a larger hunk of organic plant matter in the form of an oak tree. A marble statue of Zeus is composed of marble matter in the form of a representation of Zeus. Form plus matter equals a marble statue of Zeus. Simple.

The third cause is the efficient cause. With the efficient cause, Aristotle is describing the external processes that affect various objects that produce various effects in objects. Effect is the root of the word “efficient,” which means an immediate agent that produces an effect. An efficient cause will only affect objects that have the potential to be actualized by that cause. For example, fire only acts as an efficient cause on objects that have the potential to be changed by fire. Fire will not cause a metal fork to change, but fire can cause an object that has the potential to burn to catch fire. In that case, there is a transfer of actuality to an object that has a specific potential to be so actualized.

Aristotle’s fourth cause is the final cause. The efficient causes that affect objects go hand in hand with an object’s final cause. The fourth cause is final in the sense that it has a destination (although not necessarily a termination). What is the end or purpose for any object? What is the function that it is meant to fill? That is the object’s final cause—the end for the sake of which a thing has been constructed or has come to be. In Aristotle’s writings, there is a strong sense that everything has a purpose, that everything is working toward its own end or goal. The final cause is an object’s goal and purpose.

Interestingly and importantly, Aristotle seems to suggest that an object has the desire to actualize its potential. That’s because, in Aristotle’s system, there is only one reason why an acorn exists—to become an oak tree. The acorn does not want to be squirrel food; it wants only to become an oak tree. The final cause is inherent in the object’s essence or form.


Medieval philosophers adopted either Plato or Aristotle’s theories of knowledge and reality. Some followed Plato in seeing the intelligible realm and the Universals to be the proper object of our rational activities. Others followed Aristotle’s idea that every object in the world is striving to fulfil its purpose. Despite Aristotle’s idea that we come to understand objects in the world through observing them and cataloging their behavior, medieval scholars focused on Aristotle’s idea of objects having final causes. Through several accidents of history leading to certain documents of Plato and Aristotle being available, medieval philosophers attempted to synthesize Plato’s concept of Universals with Aristotle’s concept of final cases. Early modern philosophy, which emerged in the 1600s, was a reaction against medieval Scholasticism, which attempted to synthesize Plato and Aristotle, in particular, trying to reconcile the concepts of the intellectual realm and final causes.


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