Plato’s definition of what knowledge is, ‘justified true belief’, has been trotted out by countless IB students for generations, often without much thought as to its genuine value or validity as a definition of what knowledge is. However, what are we to make of the fact that the definition includes the term belief? What does the definition actually mean? Is it not possible to interpret the definition in different ways? And has anyone since Plato not provided a better one? Let us explore these questions and see what transpires.

By ‘justified true belief’ I take Plato to have meant that knowledge requires the coming together of three factors. Firstly, belief is necessary for unless you positively accept as certain what you think you know, there remains an element of doubt and as long as doubt persists you cannot honestly say ‘I know.’ Knowledge therefore requires belief in your own certainty. Some of course would argue that the experience of knowledge is clearly distinct from that of belief and therefore to include the latter in a definition of the former is to muddle two separate philosophical categories. I leave you to decide for yourselves as to the validity of that argument.

Your belief, according to Plato, must also be true. Truth is of course a notoriously slippery idea and philosophers keep themselves busy by continuing to come up with ever new and exciting ways of capturing the essence of this concept. The standard models for truth are the coherence, correspondence and pragmatic theories. The first claims that for something to be true it has to be consistent with the rules of logic and of accepted sets of beliefs – it has therefore to hold together, to be able to sustain attacks from critical thinking, to cohere (to be logically consistent). The second states that for something to be true it has to correspond to a real state of affairs. In other words it has to accurately represent an objective reality, to capture a fact or set of facts. Immanuel Kant of course doubted whether we could really know ‘things in themselves’, so to be able to verify whether a truth is also a fact may prove to be more elusive than the correspondence theory suggests. Be that as it may, correspondence is what most of us operate with every moment of every day so it has its uses. Lastly and largely as the result of the shortcomings of coherence and correspondence, it was suggested that what is true is simply whatever works. This has a lot to recommend it for it simply asks whether the truth considered actually fulfills any practical purpose, in other words the pragmatist merely asks, does it work? If it does, it is true. Pragmatists are eminently sensible people and therefore are not wasting time on debating esoteric concepts the truth of which can never be absolutely established.  So here we have it, truth in all its glory, up to you now to decide which you’ll plump for.


For knowledge to obtain, Plato says you have to believe it is certain and it has to be true. The final piece in the edifice is that this true belief must also be justified. In other words evidence must be put forward to demonstrate that the ‘knowledge’ you are claiming isn’t just a figment of your imagination. By evidence I take him to have meant either some concrete or material evidence, or some logical reasons to  show that your are justified in holding this true belief. Strangely enough we seem to be back with the issue of what counts as true or not. For concrete evidence, you could read the correspondence theory of truth, and for logical reasons you could read the coherence theory of truth. However, Plato was surely right in insisting that any knowledge claim had to be supported by evidence of some kind and for many this is the essence of what is meant by knowledge. Anything which is asserted without ‘proof’ is nothing more than an act of faith and can never count as being reliable knowledge. Knowledge is indeed ‘justified true belief’ then, Plato was right and that’s that.

If I may interrupt the celebrations for a moment, is it really so that Plato had knowledge licked? It seems to me that it is perfectly possible to deduce from his definition that knowledge is nothing more than a belief in my belief. Let me explain, we have seen that knowledge requires belief in the certainty of what is claimed. It also requires belief in the truth of the claim. Whichever model of truth you use you have to believe in its power to lead you to the truth. Finally, you have also to believe that the evidence or the justifications you provide cannot be challenged. As long as there is the possibility of doubt (see Descartes) you cannot claim to know that your evidence justifies your claim. In short it is my contention that, unpacked, Plato’s definition  of knowledge amounts to no more than this; namely that to know is ‘to believe in a believed belief’, but maybe this is the best any of us can do.