Three Types of Knowledge and Justified True Belief (JTB)

Epistemology is the branch of philosophy that focuses on knowledge and justified belief. There are at least three different types of knowledge that epistemology involves and can be expressed in the following three sentences (1):

S1. I know the tree in front of me. 
S2. I know how to play football. 
S3. I know that Nelson Mandela was a South African president.

What are the kinds of knowledge expressed in each sentence? The first sentence (S1) expresses knowledge by acquaintance. One knows something in that the object of knowledge is directly present to one’s consciousness. John knows that the tree is in front of him because he sees the tree. He is directly aware of it through sensory intuition. Intuition here is understood not to mean a guess or irrational hunch, but rather a firsthand awareness of something that is directly present to consciousness. Many philosophers believe that we know various things by acquaintance or intuition; for example, we know our own mental states like thoughts, feelings, and sensations; we know physical objects through our five senses; and we know mathematical truths in that we can simply “see” that 3 + 2 = 5. We can “see” this because we seem to have a type of intuitional form of awareness or perception of abstract, immaterial objects and the relationships among them (e.g. numbers, mathematical relations, propositions, the laws of logic, etc.). Arguably, all of these examples of knowledge are cases of knowledge by acquaintance. 

S2 is a form of knowledge that philosophers term “know-how”. This form of knowledge is one’s ability or skill to act in a certain way and perform some task or behaviour. As such, a person can “know” how to play football, speak Mandarin, drive a car, etc. Furthermore, know-how does not always involve conscious awareness. A person can know how to perform some task or behaviour through repeated practice to the point where he is performing the activity in question without being consciously aware.

S3 has been called knowledge by description or propositional knowledge. Here someone knows that P where P is a proposition. A proposition is defined as the content of a sentence or statement.

Justified True Belief

In his dialogue Theaetetus, Plato presented what is known as the standard definition of propositional knowledge, which is justified true belief (abbreviated as JTB). On this definition, if a person knows something, then what he knows must be true. It does not make sense for John to say that he knows the dog is on the yard if this is a false belief and the dog happens to be inside the house. So a necessary condition of knowledge is that what is known is true. But philosophers argue that truth is not sufficient for knowledge. There are many truths that no one has ever thought of, much less known, and there are some truths that someone may think about but not know. On the JTB view of knowledge, a second element is belief. If John claims to know something in the propositional sense, he must at least believe it. It does not make sense for John to say that the dog is in the yard but not believe the dog is in the yard. So belief is a necessary condition for knowledge. Yet belief is also not sufficient for knowledge. People believe many things that they do not know to be true.

Thus true belief is a necessary, but not sufficient, condition for knowledge (2). This is because John can come to believe or hold to beliefs that are true but yet have no justification or warrant for those beliefs. It is possible that John might believe something is true simply by chance or accident. For example, a belief such as “It is raining in Johannesburg right now” could pop into John’s mind. John really believes this thought and, by pure chance, it is in fact raining in Johannesburg at this time. John would have a true belief, yet he would not have knowledge of the proposition in question. What John lacks is justification for the belief that it is raining in Johannesburg right now. John can be said to have justified belief if he has sufficient evidence for the belief, has formed and maintained the belief in a reliable way (such as on the basis of his senses or expert testimony and not by palm reading), and his intellectual and sensory faculties were functioning properly in a good intellectual environment when he formed the belief.

The Gettier-Type Counterexamples

The JTB definition was, with the exception of a few objectors, the accepted definition of propositional knowledge until 1963 when Edmund Gettier published a paper criticizing it. Gettier’s paper presented two short counterexamples showing that while JTB may be necessary for knowledge it is also not sufficient. Since Gettier’s paper, various counterexamples have been raised confronting the standard JTB definition of knowledge. Here is a Getter-type counterexample that shows JTB not to be sufficient for knowledge:

“Suppose Fred believes that his wife Betty is at work and the basis for this belief is the fact that he has just seen her leave for work thirty minutes ago, she always goes directly to work every day, and she had told him as she left that she was going straight to work because she had a busy day ahead.  However, suppose further that, in reality, Betty was fooling Fred and, instead of driving to work, she went to a clothing store to get Fred a new suit. On arrival at the store, Betty was kidnapped by her friends and taken to work for a surprise birthday party for Betty” (3).

According to this example, Fred’s belief that Betty is at work is a justified true belief, but it would not seem to be something that Fred knows. Many philosophers now agree that JTB is not sufficient for knowledge and that its three elements too weak or broad. This has led to interesting work being done in this area. Some philosophers have attempted to uphold JTB by showing that the Gettier-type examples do not work because the people in them didn’t really have justification. Other philosophers have accepted the counterexamples and maintain that JTB is necessary but not sufficient for knowledge and look for a fourth condition. A third strategy is to accept the counterexamples and reject the tripartite analysis of knowledge but replacing “justification” with something else to form a new tripartite definition of knowledge.


  1. Craig, Willian Lane., and Moreland, J. P. 2009. Philosophical Foundations for a Christian Worldview. Westmont: InterVarsity Press. p. 157-160.
  2. Craig, Willian Lane., and Moreland, J. P. 2009. Ibid. p. 160.
  3. Craig, Willian Lane., and Moreland, J. P. 2009. Ibid. p. 162.

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