Justified True Belief

By Michael Gakuran | | Philosophy | Leave a comment |

jtb-account-knowledgeAs we saw last time, the standard Closure Argument for knowledge seems to hold some problems. Indeed, it even gives fuel to the skeptic in her attempts to shake us up in her claim that we can’t know a great many of the things we think we do.

Philosophers have separated knowledge into different types. We will be focussing on what is known as ‘propositional knowledge’ – declarative sentences such as the type ‘2 + 3 = 8’ or ‘A crow is a bird’. Notice that a proposition can either be true or false – it doesn’t actually have to express a fact. There are also different types of propositional knowledge known as a priori and a posteriori (basically, knowledge independent of experience and knowledge dependent on experience). I’ll go into these later on.

So then, let’s take a look at what philosophers have traditionally taken to be the definitive tripartite account of knowledge. It’s known as the ‘Justified True Belief’ account, or just JTB for short. It has 3 parts:


I cannot know that p unless I believe that p.

Intuitively, knowledge seems to be some kind of belief. If you don’t believe in something, can you really say you have knowledge about it?

Perhaps it seems like you can know something you don’t believe in? For example: ‘I don’t believe in war!’ I know what war is but I don’t believe in it, right? Not quite, in this sense you are saying that you don’t agree with going to war, or the use of war to resolve problems (etc). This is not the same as saying you don’t hold a belief about war. It seems like you have to have a belief about war to be able to say you don’t agree with it.

But some philosophers have argued that you can know something without believing it. Imagine the situation where you really don’t believe you know what 9×7 is. You believe you have forgotten the multiplication tables you learnt at school. But when asked, you instantly reply with the correct answer ’63’. You do this with many other examples too, proving you weren’t just lucky. You actually knew what the answer was (without having to work it out in your head) because you had the multiplication tables drilled into your head when you were younger, but yet you didn’t believe you knew. Hence we have an example of knowledge without belief, right?

Again, not quite. One could argue that it is isn’t even a case of knowledge to begin with. Even if you did believe that 9×7=63, your belief wouldn’t be justified. In order to justify it, for example, you could prove 9×7=63 by writing down the 9x tables, or be told be a mathematician that you are correct. In which case you would again come to believe 9×7=63.

The point is that, when asked, you had no justification for your being able to answer the question ‘what is 9×7?’ You didn’t believe it was right because you felt you had forgotten, and you couldn’t justify your answer without being told you were correct or working it out for yourself (in which case you would also come to believe the fact again). Hence without justification, this wouldn’t be a true case of ‘knowledge’. I talk more about justification soon.


I cannot know that p unless it’s true that p.

We are sometimes mistaken in what we believe. Our beliefs turn out to be false. ‘Knowledge’ then, could be said to be our collection of true beliefs. The truth condition for knowledge is not usually disputed as most people agree; it seems very unlikely that we can call something ‘knowledge’ if it is false.

A small point to consider however, is that false things can contribute to our understanding of the world. I may know something not to be true (I have a belief that it isn’t true) such as I believe the statement ‘the world is flat’ is false. While it is the case that this is a claim about a falsehood, something more seems to be going on. When I say ‘the world is flat’ is false, what I am doing is tacitly promoting a different picture of the world. So in saying ‘the world is flat’ is false, I am adding further strength to the argument ‘the world is round’ or some other such claim. In this sense ‘the world is flat’ is false could be said to have elements of truth.


I cannot know that p unless I am justified in believing that p.

Why justify our true beliefs? The simple reason is because lucky true beliefs don’t seem to count as real knowledge. For example, I may believe that my friends have planned a surprise party for me as I have been feeling a little down recently. (I have a strong feeling that I am in for some good returns after all my hard work.) I return home that day and open the door only to be met by party poppers and music with all my friends grinning. It turns out that my belief was true, but would we say I knew that my friends had planned a party for me?

Most philosophers would answer ‘no’. Epistemic luck (knowing something out of luck) does not count as knowledge. So, to avoid these sorts of lucky (or bad-luck) cases, we need a third condition for knowledge. Justification. We need some sort of evidence or good overall epistemic reason to believe something.

Notice as well that justification is fallible. We may have a justified belief that turns out to be false. For example, the weatherman tells me there is a 90% chance of a fog settling over my town in the evening, so I believe him and return home early to avoid it. It turns out that the fog did settle at all. I now have a false belief which was justified. But this is nothing to worry about – humans are very fallible creatures! We often make mistakes, so if anything this point adds weight to the theory. It is practical.

As you might guess, it is this premise – justification – that is most controversial. How do we define what counts as evidence to justify something? And how much? What method(s) can we use? I’ll examine this more in subsequent articles.


So finally we have our argument as follows:

Subject (S) knows something (p) iff:

(P1) p is true
(P2) S believes that p
(P3) S is justified in believing that p.

Note: ‘iff’ means ‘if and only if’

These are the traditional 3, ‘individually necessary but jointly sufficient’ conditions for knowledge. As we will see however, a philosopher by the name of Edmund Gettier provided a very succinct but devastating argument to show the flaw in our brilliant theory. Onto Gettier Cases…

Sources: Lecture handouts written by my lecturer Daniel Elstein, The Internet Encyclopedia of Philosophy and The Stanford Encyclopedia of Philosophy

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