In a dialogue called the Theaetetus, Plato provided perhaps the first, and certainly the most influential, account of knowledge in western philosophy. In it, he asserted that knowledge consists of three components.
What kind of thing is knowledge? To answer, let's consider an example. We know that the Titanic was over 100 feet long. However, this knowledge is something that exists in our minds, not in the world. It must be true, because we know that the Titanic was over 100 feet long, but not everyone knows this. There are people who don’t know what the Titanic was. There are many people who don’t know the English measuring system, and don’t know what 100 feet is. However, since the Titanic was over 100 feet long independent of whether a person or persons know it or not, then knowledge must be distinct from the object itself. Knowledge is in our minds as a posture we hold toward the statement, “The Titanic was over 100 feet long.” Knowledge is a type of belief. This was Plato’s first component of knowledge (“belief” is the term used now, but Socrates and Plato sometimes used “opinion” instead).
However, people have believed lots of things. 1,000 years ago, people believed that the sun revolved around the earth. Before that, some believed that the earth was flat. Children believe that Santa Claus exists. Believers, however, don’t know these things. They can’t know them. They may think that they know them, but they are wrong in thinking so. You cannot know that which is false. Therefore, Plato’s second component of knowledge is truth. Knowledge must be a true belief (or true opinion).
However, this definition of knowledge is still insufficient. For example, suppose you are at a party chatting with a stranger who makes a request. He puts his hands behind his back and asks you to guess (i.e., to form a belief about) how many fingers he is holding up. Arbitrarily, you guess two. He takes his hand from behind his back and shows you that he is holding up two fingers. In this case, you had a true belief, but you didn’t know that he was holding up two fingers. Therefore, having a true belief falls short of knowledge. Something is missing.
There are many things we can imagine that might provide us with knowledge. Imagine if the stranger had been standing by a bar, above which hung a large mirror, enabling you to view a reflection of his hand behind his back. Or imagine that you have seen him ask the same question of everyone else at the party and noticed that he always holds up two fingers. Or perhaps your friend is standing behind him and signals you that he is holding up two. What do all of these possibilities have in common? They all provide good reasons to believe that he is holding up two fingers.
Imagine that you threw a dart at a dartboard and it stuck in the "two" area of the board. You used the dart's landing location as your answer to the stranger when he asked how many fingers he was holding up. This is a reason to answer "two," but is it a good reason? No, it is not. Does the fact that the dart landed on two enable you to claim that you knew he was holding up two fingers? No.
Note what these two examples teach us. When we have good reasons for our true belief, we say we know it. When we have bad reasons, we don’t (we can't) say we know it, even if it happens to be true.
Plato’s third component of knowledge is called justification. This term indicates situations in which we have good reasons for our belief. (Note that, when talking about knowledge, justification always applies to beliefs, not actions.) What makes a reason “good” is one of the main focuses of the field of epistemology.
Plato's Components of Knowledge
Knowledge is a type of belief
Knowledge must be a true belief
Knowledge is a justified true belief
How can we justify a belief? Consider the justifications included in the examples above: seeing the mirror, being signaled by a friend, watching for patterns, etc. In broad terms, what they all have in common is that they are grounded in the world. Justification is tied to seeing the world as it is, properly relating a belief to the world as it exists. When you properly connect a belief to the world as it is, that belief is justified.
Therefore, in order to justify a belief, one must access the world as it really is. For Plato, this meant knowing the essence of things. In order to say, “I know Jen is a human,” the speaker must know what a human truly is (i.e., what makes a human, human: its essence). This is true of essences generally, but Plato developed a nuanced and influential view of essences. It is called Plato's doctrine of the Forms or Ideas, or the Platonic Forms (initial capitalization is used when referring to this concept).
In Plato's view, essences are real entities. A real thing that is the essence of goodness, called the Form of Goodness, exists. The same is true of Justice, Humanity, and other important essences. These essences exist not in the world, but in an intellectual realm sometimes called Platonic Heaven (because it is analogous to the Christian heaven in many ways, including its lack of a physical location). Everything that is good is good because it is related to this essence, the Form of Goodness.
It may seem strange to think that there is a Form of Goodness that is everywhere/nowhere in Platonic Heaven. But let’s consider something more familiar. Do you believe in the law of gravity? If so, where is it? Everywhere? Nowhere? Platonic Heaven? All we know is that, when something is dropped, we expect it to fall. It always has, always does, always will.
Belief in the law of gravity is a standard, everyday belief. But if you replace “law of gravity” with “Form of Justice,” you may begin to realize that, just as there is something that makes dropped objects fall, there is something that makes a just act just. This "something" is the Platonic Form.
If there are such entities, they will play an important role in our theory of knowledge. Return to the law of gravity analogy. If we can accurately describe the law of gravity, then we can also know the way an object behaves when it is dropped. If we know the law of gravity, then our belief about what will happen when an object is dropped is justified. Similarly, if we can describe the Form of Justice accurately, then we can be justified in beliefs that particular acts, policies, or governments are just or unjust. If we know the Form of Humanity, then we can be justified in believing that someone is or is not human (and what makes a good human.)
Since Platonic Forms are the metaphysical grounding of reality, knowledge of reality is grounded in knowledge of the Forms. The Forms are the entities through which all knowledge comes. If we can access them and know their true nature, then we are justified in any beliefs to which they apply.