Plato is considered by Russell to be the most influential of all philosophers. This is partly due to the significant influence he had on the second most influential philosopher, Aristotle. He was born in the late 5^th century into an aristocratic family and was a student of Socrates. As a young man he experienced the defeat of Athens in the Peloponnesian War and the execution of Socrates, and in part he blamed Athenian democracy for both events, which contributed to a predisposition to the aristocratic Spartan model of society. The influence of several preceding philosophers can be found in his work.

Knowledge and Opinion

Plato’s theory of ideas is perhaps his most famous and influential contribution to philosophy. Unfortunately it is also one of the most difficult to understand, and I won’t pretend to have understood it fully. Plato’s philosophy rests on Parmenides’ logical distinction between reality and appearance, and a Pythagorean mystical and religious tone about reality. A philosopher is one who achieves absolute knowledge rather than something that is just opinion. Knowledge is infallible and is about things which exist. However opinion is about things that simultaneously do and do not exist. For example a person may say that an object is beautiful and another that it is ugly. The beauty of the object is therefore opinion rather than knowledge. All objects can be perceived differently to different people, so in Plato’s strict sense they do not really exist, or rather what we perceive is not what really exists.

We have general words we don’t apply to specific things. The word ‘cat’ does not apply to a specific cat but all animals that we consider to be cats, and the idea of a cat is eternal. The idea of a cat exists because we can have infallible knowledge of that idea. Each individual cat is an imperfect version of the idea of a cat, and so the question of what it is is a matter of opinion rather than knowledge. What we perceive when we see a cat is not exactly a cat, in that it is not the same as the idea of a cat. When things have a common name (such as ‘cat’) they also have a common idea. Any particular thing is just a copy of the idea. Plato adds the religious element by saying that the ideas of things are not just thoughts, but are real things created by God, which we can perceive if we have knowledge. Philosophers are only interested in the divine original things created by God about which they can achieve knowledge, not the mundane copies of these things, which are subject to opinion.

Plato distinguishes the world of the intellect where there is knowledge and the world of the senses where there is only opinion. There are two kinds of intellect: ‘reason’ and ‘understanding’. Understanding is the kind of intellect used in mathematics. It is inferior to reason as it relies on hypotheses which it cannot test. For example in geometry we may start by saying that ABC is a rectilinear triangle. We cannot ask if that initial statement is true and we can’t create a rectilinear triangle because we can’t draw perfectly straight lines. Mathematics then is limited to hypothetical truth. Reason is required to gain knowledge of the idea of perfectly straight lines, from which geometric propositions can be affirmed categorically rather than just hypothetically.

The Allegory of the Cave

Plato explains the difference between the clarity of the intellectual world and the blurred world of the senses with the famous allegory of the cave. Using the analogy of sight, we only see things clearly when light shines on them. In twilight we see partially and in darkness not at all. A mastery of philosophy represents the light which allows us to have knowledge where otherwise we would be fumbling from one opinion to the next. Imagine then a cave where people have been held in chains since childhood, facing the back of the cave and unable to turn their heads. There is a fire burning behind them, and whenever people walk in front of the cave they cast shadows on the wall of the cave. The people in the cave have never seen those who cause the shadows and so assume that the shadows are real, rather than the poor imitations of people. When someone finally escapes the cave and comes into the light they perceive the real objects and realise they have been deceived by shadows. The escapee then returns to the cave to explain what they have learnt to the others. Unfortunately their eyes have become acclimatised to the light and they perceive the shadows less clearly than the others, and so appear to them less knowledgeable.

Plato’s theory of ideas is underpinned by the belief that true knowledge can only apply to the abstract – to ideas, not to tangible objects. In his work ‘Theaetetus’ Plato argues that there is no such thing as empirical knowledge. Russell organises Plato’s thoughts into three arguments. The first is the limitations of our senses. We can sense colour through our eyes and sounds through our ears, but we have no organs to sense characteristics such as good and bad, or to establish what exists and what doesn’t. The second is the individual nature of our perceptions – since we can all perceive things differently, what we perceive must be opinion rather than true knowledge. The third argument is the idea that what we perceive is constantly changing (if only very slowly), therefore any ‘knowledge’ we gain through perception is immediately out of date. While it is tempting to disregard these arguments out of hand, we might at least concede that not all knowledge can be gained through empirical means. 

Clearly Plato’s theory of ideas has issues, but Russell reminds us to respect its originality. The distinction between what is real and what is appearance cannot be as dramatic as Plato insists. Whatever we perceive must in some sense exist, even if we don’t always perceive every aspect of things. Also to say that an object is both beautiful and ugly is not necessarily a contradiction, and so isn’t a barrier to existence. For example it could be that some aspects of the object are beautiful and other aspects are ugly. In addition Plato says that ideas are timeless but also that they are created by God, so must have been created at a point in time. Further, where did the idea that God created come from before it existed? However Plato became aware of some of the flaws in his theory of ideas, and deserves great credit for exploring those flaws in his work ‘Parmenides’ in later life.