One of Plato’s most famous theories is related to knowing — the Theory Of Forms. The basic idea is that we understand the world around us through our senses, but we are only seeing shadows or representations of unchanging, eternal ‘essences’ that are known as forms. These forms are the true reality and all genuine knowledge is knowledge of those forms. 

The idea of the theory of the forms come from Plato’s Republic, with his parable of the cave:

[Socrates] And now, I said, let me show in a figure how far our nature is enlightened or unenlightened: — Behold! human beings living in a underground cave, which has a mouth open towards the light and reaching all along the cave; here they have been from their childhood, and have their legs and necks chained so that they cannot move, and can only see before them, being prevented by the chains from turning round their heads. Above and behind them a fire is blazing at a distance, and between the fire and the prisoners there is a raised way; and you will see, if you look, a low wall built along the way, like the screen which marionette players have in front of them, over which they show the puppets….they see only their own shadows, or the shadows of one another, which the fire throws on the opposite wall of the cave… what will naturally follow if the prisoners are released and disabused of their error. 

At first, when any of them is liberated and compelled suddenly to stand up and turn his neck round and walk and look towards the light, he will suffer sharp pains; the glare will distress him, and he will be unable to see the realities of which in his former state he had seen the shadows; and then conceive some one saying to him, that what he saw before was an illusion, but that now, when he is approaching nearer to being and his eye is turned towards more real existence, he has a clearer vision, -what will be his reply? And you may further imagine that his instructor is pointing to the objects as they pass and requiring him to name them, -will he not be perplexed? Will he not fancy that the shadows which he formerly saw are truer than the objects which are now shown to him?

This theory, according to Plato, is the basis of things around us — there is an essence of ‘dog-ness’ that is behind all kinds of dogs, for example. We can identify a square, because there is a true, eternal square that is the basis of all squares in an imperfect world. We are only seeing the shadows of the puppets, while chained in a cave.

Of course, this has issues with where these perfect forms are, and it does seem like a simple solution to making a basis for knowing things and even how to behave (is there a ‘perfect form’ of a human being, with perfect behaviour as well?). However, the theory does provide us with an interesting proposal on how do we trust our senses and if there are standards or definitions for things, which we then build upon.