Theaetetus by Plato

"If you don't know where you're going, you'll end up someplace else"

Yogi Berra

Plato's dialogue Theaetetus takes on what we call today epistemology, the theory of knowledge. Socrates, Theaetetus, and Theodorus attempt to answer the question "what is knowledge?."

Put forward first is the doctrine that perception is knowledge. This is what the sophist Protagoras taught, that "man is the measure of all things." What is true for me is true for me, and what is true for you is true for you. If you feel cold and I feel hot, we are both right because we feel so. I may have a fever and you do not. In our own situations, we are both correct about the states we are individually in. 

This is rejected as a basis of knowledge because it is contradictory that we are both right. It is true that what everyone says is true? Then how can anything be wrong? If nothing is wrong, then that doesn't give us a meaningful definition of what knowledge is. People have contradictory opinions which cannot be reconciled. God exists and doesn't exist? You may believe God doesn't exist and I do. But we both cannot both right. We are in this case talking about the same thing which cannot both be and not be. 

171a " In saying that everyone believes what is the case, he is conceding the truth of beliefs which oppose his own; in other words, he is conceding the truth of the opinion that he is wrong"

The dialogue also takes on the Heraclitean philosophy that everything is in flux, and things can both be and not be. Everything is becoming, not being or not being. Everything becomes its opposite and is defined by its opposite. Hot things become cold and living things die. This we perceive all around us. What is true at one time is not true another moment. Today this doctrine is called moral relativism. Just look at how slavery used to be acceptable 150 years ago, and is still practiced in certain parts of the world. There is a right and wrong, but this is always changing. It is different for every culture and time period. The Heraclitean philosophy accounted for opposites like truth and falsehood, but argued that opposites were co-dependent and therefore constantly changing. 

The problem with this doctrine is that if everything is changing, it is not perceived. For how can we perceive what not is? The "negation" of something is relative and requires some basis of what either is or was. It isn't perception if not of something. 

182e "So if everything is changing in all respects, we shouldn't talk of seeing something rather than not seeing it, and any other perception is no more perception than not perception

But perception is not knowledge either as said before. Rather it is reason which distinguishes what something from what it isn't. It is true that things change, but also that we have an idea of what something is even if it has changed. If not, how could we come to know change? Also, memory stores knowledge of a past perception of something which as the Heracliteans would say is no more. Yet we still have a notion of this thing which no longer is what it was. 

186d "Therefore knowledge is not located in immediate experience, but in reasoning about it, since the latter ,apparently, but not the former, makes it possible to grasp being and truth."

Now the conversation turns to a new definition of knowledge: true belief. What we hold in the mind about being reflects what the thing we think about actually is. Belief is an internal affirmation about something which exists. A statement about ourselves, related to something existing. A true belief corresponds to its object. This definition of knowledge admits truth and falsehood which corresponds to something independently existing.

The issue here is how to distinguish between true and false belief. It isn't as if the person who has a false belief knows they are wrong. Otherwise it wouldn't be false to that person. It is either that said person knows what is true and false and confuses them, or they know neither. But how could someone confuse what is true and false, without already knowing what is true and false? How could they personally believe in what they know to be wrong? I'm speaking consciously here. This obviously is absurd.

199d "although knowledge is present, it is possible for the mind to know nothing and be ignorant of everything"

The final attempt to provide a definition of knowledge is justified true belief or true belief with an account. What is true surely has defining features to it. A whole is not more than its parts. So if we can determine the "markers" of something, we have knowledge of it. Yet, 203b "how can one say what the elements are?" If something is what it is, as a single thing, then how can any one thing be an indicator of it, unless it is the thing we're looking for? Can one know the elements without already knowing what they belong to? And if they belong exclusively to that, then do we already know what we're looking for? 204a "Because where there are parts, the whole is identical to the parts." 

The answer to this dilemma is that one can only know to be true and false what one already knows to be true and false. There needs to prior knowledge in the mind, which has as its source something actually existing. For the idea to be true about the external world must have some objective basis in this real world. 

209d "we still need correct belief about the uniqueness of things whose uniqueness we already have correct belief about!"

209e "we require something we already have, in order to get know what we believe."

This is why according to Plato we need a innate ideas. That always have their basis in reality. In that they derive from a prior experience with the object, or are built into our minds from birth. Or that our minds have access to a non-sensual world of forms which allows us to recognize their transient forms in the ever-changing universe described by Heraclitus.