Philosophy is concerned with one important question which is “what can I know?” From our acquaintance with the Sophists, we know that there are philosophers who think knowledge is impossible.  We also know, however, that there was one philosopher who was not certain about the impossibility of knowledge, and his name was Socrates.  One reason Socrates was uneasy about the claims that knowledge was impossible is that they were at least certain about one thing: the impossibility of knowledge.  Socrates, for his part, wanted to know how they knew this; his own claims to knowledge about such matters were more modest.

But he himself had no theory on his own part to resolve this problem. It is only until Plato do we find a philosophical theory which attempts to defeat scepticism and give a firm foundation to knowledge.  But Plato could only offer an answer to this problem, because Socrates kept this question alive, by refusing to accept the certainty of the thorough going scepticism of his contemporaries.

What do we ordinarily mean when we say that we know something? Do we not mean that we have acquaintance with that thing? But what do we mean by acquaintance? Commonsensically, we mean that we perceive that thing. We see it and we know that it is there. But ‘knowing,’ explained in this manner really would open us up to the arguments of the sceptic. It precisely because the Sophists had this conception of knowledge, as perception, that they were sceptical about the possibility of knowledge. Why is perception not the most secure foundation for knowledge? Surely it is the clearest kind of acquaintance with things? First of all, we know that we can be deceived by our senses. What seems far away can be small, when in fact it is large. Or perception can play tricks upon us to make us think there is something there when in fact there is not. More importantly however, perceptual knowledge can only be subjective. What appears hot to you might appear cold to me. For example, you might go outside and then come in again and the objects will appear to you warmer than they appear to me. Obviously you cannot doubt these perceptions, but they cannot be said to be truly knowledge of the object, for who is to decide between your perception and mine? Which is the truer perception?

It would appear that truth and perception cannot go together. Perception and certainty can, but this is not the same as truth. When we say that we know the truth of something, we mean that we know what it really is, not just how it appears to me. It would seem that the sceptics would be correct to say therefore that true knowledge is impossible. Plato’s argument is not to say that the sceptics give a false description of perception, but that they miss something out of their description of the act of perception, which does not belong to perception as something sensible. Perception already includes within itself that which exceeds perception, and it is there where the true ground of knowledge is to be found.

Thus just as much as the Sophists, Plato takes our everyday experience of the world and tries to describe actually what is going on when we say that we know something. The problem with the simply model of judgement is that it describes our knowledge as merely being a sensation of objects, but of course we do not just sense objects as collections of subjective sense-data, but make judgements about them. I say ‘I see a chair’. My ability to make this judgement means that I have already gone beyond the mere act of perception. Sensibility, which sees our relation to objects merely in their immediate presentation, cannot give an account of knowledge. How is this case? To answer this question we have to look in more detail at Plato's argument as it is presented in the Theaetetus.

In the Theaetetus, Plato begins with the common sense understanding of what knowledge is as perception. Socrates asks his interlocutor, Theaetetus, what he thinks knowledge is. He answers as follows:

Now Socrates replies to this definition by saying that it reminds him of what Protagoras used to say and which is stressed as the cornerstone of sophistical thought, namely ‘Man is the measure of all things’. How is it that this expression can be equated with Theaetetus’ definition of what knowledge is? Socrates takes Protagoras’ statement to mean that all knowledge is subjective. We can see why this must be so, as we have said, when we identify knowing something with merely perceiving it. In this case, I am the measure of how something appears to me.

Is it not true that sometimes, when the same wind blows, one of us feels cold and the other does not? Or one feels slightly and the other exceedingly cold? ... Then in that case, shall we accept Protagoras’ saying that it is cold for him who feels cold and not for him who does not?[3]

When we perceive something, what we mean is that the thing seems to have such and such properties for me, and it seems to have different properties for you. What we do not mean is that either of us actually have knowledge of the thing as it is in itself. Seeming means that x appears to me through such and such perceived properties. A perception, therefore can only know how something seems to appear to a particular person and what a thing “is” is reduced to how that thing appears to that person.

How does this interpretation of knowing something affect our understanding of truth? For a start immediate perception seems to be that which is the most true and certain, since I cannot doubt it. I may be doubtful about whether God exists or not, but I cannot doubt that I see X when I see it (I might doubt its existence or that anyone else can see it or even that what I see is really what is there, but that I see X is absolutely self-certain) When I say I see something and say it is for example ‘black for me’, then this statement must be true, because every perception as long as one accepts that it is only a ‘seeming’ must be true. Moreover this conception of knowledge seems to coincide with reality itself. Is it not true to say that everything is in movement and changing? For example something at time t1 can appear to be hot, whereas at t2 it can appear to be cold. With perception, it is true to say that everything must be in flux and truth something subjective.

Plato does not disagree with this, rather he disagrees that this is the complete description of what we mean by knowledge and that if we take it as the absolute description of what is, rather than merely the description of the act of perception in a restricted sense, as sensation, we will be lead to absurdities. What this picture leaves out is that we do not merely perceive the world, but we use concepts to describe it, and concepts are not themselves something that can be perceived.

To understand the difference between a perception and a concept means understanding the difference between white as a property of a thing and whiteness itself.  Innumerable things can be white, but nothing can be whiteness itself. I perceive a white stick or a white stone, but I do not perceive whiteness itself and yet I do know what whiteness means, because I understand what the word expresses, even though I have never seen whiteness itself. Now it is upon these general concepts that my judgements are based not upon my subjective perceptions and it is because of these general concepts that we have a conception of truth and falsity, because we have an objective measure of what might be right and wrong. But what evidence for these concepts do I have? The evidence is what I actually do. For although it seems common sense to say that my knowledge about the world is through the senses, this world would in fact be meaningless.  The point is I do not just perceive the world, but I reflect upon the perceptions that I have about the world, and I say ‘this stone is white’ or this ‘stick is white’. No animal perceives in this way simply because that is all it does perceive. It does not see as stick as a stick, nor a stone as a stone, nor white as whiteness, for it does not have the linguistic ability to have learnt the concepts of whiteness ‘stoniness’ or ‘stickness’.

There is, therefore, a great difference between merely perceiving something and knowing something. To know something is to be able to grasp which concept is applicable to it, and unlike perception, which in itself is infallible, for I always see what I see, I can make a false judgement or I can understand the concept itself wrongly; not understanding, for example, the concept of ‘triangle’, means a three-sided figure (the other thing about conceptual knowledge is that I can know this without having to perceive a triangle). Animals look out onto the world with their senses; Human beings with their minds.

We can think about Plato's argument in the following way. Consider a judgement of perception, ‘This is a square’. In this judgement, Plato argues, our mind is turned in two different directions at the same time. It is turned towards the perceptual object, the square if it judges correctly. But at the same time, it is turned towards the idea of ‘squareness’ (What Plato calls its Form).[6]We are turned towards this form, not with our physical eyes, of course, for they only see, the actual square, but if you like, with our ‘mind’s eye’. Both the object and the form have a real causal and therefore explanatory power. My perception of the square is caused by the actual square that I see, but the fact that I can recognise it as a square is because I apprehend or directly understand the form of squareness. The form of the square is exemplified in the particular square itself, but also the square itself can be seen as a square, because it is an instance of the form of squareness in general.

There is 20th century philosopher, Edmund Husserl, who expresses the same idea in the following way.

Once one has accepted this distinction between perceptions and sense or meaning, or concepts, then one also, at least for Plato, has to accept a certain metaphysical picture of reality. It is clear that perceptual objects and concepts do not have the same properties. A perceptual object can change. I can burn this sheet that the picture of the square was printed on and it would not longer be a square but ashes, but I cannot burn the concept of squareness. The object is mixed, for example, the square, you see is also black and the lines in which it is draw are of such and such thickness, where as the concept of squareness in neither black nor made of lines of such and such a thickness, but simple denotes the conceptual meaning of word - a four-sided figure of equal sides - you might imagine a square in your mind when you think of these words, but the meaning of these words are not determined by this picture. We all have different picture of squares in our minds, but we all think the same thing in the definition of squareness.  Lastly the being of the idea of squareness is absolute, which literally means that it is independent. It is not dependent on actual squares for its existence, whereas the squares themselves are only squares because they participate in it. There are many different squares, and there is only one idea of squareness.

This last quality of  the forms, their absolute nature, their independence from perceptible reality, is the most difficult but also the most Platonic idea. The existence of the idea of squareness is prior to the experience of squareness. I would not be able to recognise X as an square without already possessing the idea of squareness in advance prior to the act of perception.  Plato describes this anteriority of the concept in relation to the particular in the idea of a memory that precedes my birth; a memory of the collective rather than the individual.  Or better, that every individual is already collective.

This separation is taken literally by Plato. The Forms, precisely because they have different properties than the particular things that embody them exist in a different reality than the sensible perceptions. This leads Plato to a dualist metaphysics, which splits existence into the intelligible and the sensible; the former is the place of truth and being, and the latter the place of appearance and falsity. To know what something is, its being, and not merely how it appears, is to know the truth of that thing. To know what something is to know the form that it embodies. The task of philosophy is to demonstrate the existence of the intelligible realm and to show how it must differ from the sensible. This task is presented most philosophically in the Republic, but it appears in a much less abstract manner in the Symposium in Diotoma’s speech and in the Phaedrus in Socrates’s second speech about love.

Diotima tries to explain how it is possible to gain knowledge of what beauty is in itself (Not the appearance of beauty, but the true essence of what beauty is; the truth of beauty) She describes how one gains this knowledge in the form of an ascent to the “beautiful”. First of all, one has to grasp the beauty of a particular thing, in this case, the sight of a beautiful boy. The second  moment of the ascent to the true knowledge of beauty is the realisation that there are other persons which embody the same quality of being beautiful as this boy. Already at this stage, one is beginning to grasp the realisation of the concept of beauty. But the next jump is the most important step. One realises that it is not only bodies which can be beautiful but also the soul and what the soul contains, namely all the ideals, values and discoveries the human spirit, from mathematical equations to the laws that govern the constitution. Having reached this step, the final one is to realise that behind all this beautiful things, the beautiful bodies, ideas and creation is one and the same form of Beauty which they all participate in, like as though there were droplets of water in one great ocean.

It is important to realise that in this philosophical vision that all the beautiful things become the expression of the one idea, the beautiful in itself, which of course is not a thing although it is what makes all these beautiful things beautiful, and it is this idea which is the true basis of knowledge and not perception.  Moreover, because of the certainty through which we know ideas, we can now safely assert, unlike Socrates, that knowledge is possible against those who claim, because they think knowledge is perception, its impossibility.