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Knowledge (philosophy)

In philosophy, Knowledge is usually defined as beliefs that are justified, true and actionable. Any description, hypothesis, concept, theory, or principle which fits this definiton would be considered knowledge. Philosophy generally discusses propositional knowledge rather than know-how[?].

The traditional way of gaining knowledge has been by accepting the teachings of generally recognized authorities of the past. These could be philosophical, religious or scientific teachings. A second way to derive knowledge is by observation and experiment: the scientific method. (Knowledge gained by observation was ignored or rejected by many classical religious authorities.) Knowledge is also be derived by reason and logic, and by mathematics.

Table of contents

Defining knowledge

People often use the term "knowledge" in different ways, without precisely defining what they mean. As such, we must first define what this article is not about:

  • Knowledge is sometimes loosely used as a synonym for "belief"; a "belief" being an internal thought or memory which exists in one's mind. For the purpose of this article, we will generally avoid discussing beliefs in general.

  • All people hold beliefs about the world that may or may not constitute as real knowledge. For example, most people have the belief that the Earth is roughly spherical in shape; they also hold that this belief is justified, true and actionable; hence, this belief constitutes knowledge. However, most people also believe in some form of God or gods, but these beliefs have not been proved; they may even be unprovable. It is possible that they are false, yet people certainly have this beliefs within their minds, and may even consider them as knowledge. For the purpose of this article, we will not be dealing with beliefs in general.

  • Knowledge is also not "know-how" (or expertise). Knowing how to perform some practice may be useful, but that is not what we mean here by "knowledge". Knowing how to do something is different from knowing that a proposition is true. The latter is called propositional knowledge, and ways to attain propositional knowledge is one of the major points discussed in this article.

The definition of knowledge that this article deals with is "How can we tell when our beliefs are justified, true and actionable? "Justified" means that one has some evidence supporting the belief. "True" means that this belief relates to something that is predictable, for example: "one could make plans based on a true belief, and they would not fail because of the belief". "Actionable" implies that it is useful: someone can make decisions and take actions based on it.

What constitutes knowledge, certainty and truth are controversial issues. These issues are debated by philosophers, social scientists, and historians. Ludwig Wittgenstein wrote "On Certainty" - aphorisms on these concepts - exploring relationships between knowledge and certainty. A thread of his concern has become an entire field, the philosophy of action.

Inferential vs. factual knowledge

Knowledge may be factual or inferential. Factual knowledge is based on direct observation. It is still not free of uncertainty, as errors of observation or interpretation may occur, and any sense can be deceived by illusions.

Inferential knowledge is based on reasoning from facts or from other inferential knowledge such as a theory. Such knowledge may or may not be verifiable by observation or testing. For example, all knowledge of the atom is inferential knowledge. The distinction between factual knowledge and inferential knowledge has been explored by the discipline of general semantics.

Ways to obtain knowledge

All people have an ability to understand and analyze ideas, and thus to gain knowledge. However, people usually do this without thinking about the conceptual framework that allows them to do this. This framework is called an epistemology. Epistemology is the study of the origins, nature, and limits of human knowledge. All people possess undeveloped epistemologies. There are a number of different epistemological systems, discussed in the epistemology article.

One way of deriving and verifying knowledge is from tradition or from generally recognized authorities of the past, such as Aristotle. Knowledge may also be based upon the pronouncements of secular or religious authority such as the state or the church. This is known as the appeal to authority.

In Jewish, Christian and Islamic traditions, there has always been a considerable tension on the issue of authority versus experience in the formation of knowledge. Early Christian philosophy contrasted revelation from God with knowledge gained by reason. St. Augustine for instance put the knowledge of classical philosophers, especially Plato, into a Christian framework. Experimental knowledge[?] was discounted. Early Muslim philosophy, especially the Mutazilite school, medieval Jewish philosophy, and later Christian work, especially that of Thomas Aquinas, focused on Aristotle's views. These were vast controversies stretching over centuries. The (eventually dominant) Asharite school of Islamic scholars, for instance, strongly rejected most views Aristotle, while the Roman Catholic tradition generally embraced them. Such efforts to provide an ethical or spiritual basis for the foundations of knowledge continue to this day in the sociology of knowledge, Islamization of knowledge, and the many and varied strains of economics.

A logical way to gain knowledge about the physical world is through the scientific method. In this method, one starts by finding a phenomenon of interest, which generates questions. One picks a question of interest, and based on previous knowledge, develops an hypothesis. One then designs a controlled test which will allow one to test one's hypothesis against what actually occurs in the real world; predictions are made about the outcome of the test. (See scientific method for the general principles and procedures of designing, carrying, and inferring from such experiments)

Only at this point does one carry the test or experiment out; after the experiment one compares their hypothesis with the observations they revealed. If a systematic and exact correlation is found between the outcome predicted by the hypothesis and the actual outcome during the experiment, then it can be said that one has confirmed the hypothesis against the phenomenon that is investigated.

The next step is peer-review, in which one's results are distributed to others, who then review. In science, all conclusions are tentative, subject to further revision or review should new data come to light.

An hypothesis that have been shown to accurately and reliably predict and characterize some physical phenomenon, and had been sufficiently peer-reviewed and tested, may become a scientific theory.

Because of the strong establishment on real world evidence (justified), repeatability (true) and usefulness (actionable) of scientific theories. Most people regard scientific observation as the most useful and reliable source of knowledge.

Some people hold that science does not actually tell us about the physical world that they live. They hold that the world cannot be understood by science, but rather by religious revelations, mystical experience, or literary deconstructionism.

Practical limits for obtaining knowledge

What we hold to be knowledge is often derived by a combination of reason from either traditional, authoritative, or scientific sources. Many times such knowledge is not verifiable; sometimes the process of testing is prohibitively dangerous or expensive. For instance, some physics theories about the nature of the universe, such as string-theory, requite the construction of testing equipment currently beyond our technology. Since such theories are in principle subject to verification or refutation, they are scientific; since they are not proven experimentally, they are not considered certain knowledge. Rather, in such cases we have certain knowledge only of the theory, but not of what the theory describes.

The problem of justification

In philosophy, knowledge is held to be a belief that is true, actionable and justified. But how do we justify that our beliefs are true knowledge?

Justification and evidence are both epistemic features of belief. Justification and evidence are, in other words, both qualities that indicate that the belief is true. We could try out other epistemic features in the definition of knowledge, if we wanted to. Instead of "justified true belief" or "true belief with evidence," we could say that knowledge is "rational true belief" or "warranted true belief." For our purposes, the differences between these different options don't matter. The whole point is that, to be knowledge, a belief has to have some positive epistemic feature; it can't be arbitrary or random or irrational.

Philosophers have raised a number of questions with regards to these definitions, such as "What degree of justification is required for knowledge?" and "Is knowledge possible?"

What degree of justification is required for knowledge? Justification comes in degrees, from weak justification to strong justification. The better your evidence, the better justified your belief is. There is a strong sense of the word "knowledge," perhaps as used in mathematics, where you have to be either certain, or very close to certain, before you can be said to know something. The standards of knowledge there might require stronger justification than in other areas of life. Why might that be? Why would mathematicians require a higher degree of justification in order to have mathematical knowledge? Well, perhaps because it's possible to prove things in mathematics, in a way that one can't prove things about the weather or about economics or sociology. Since a higher degree of justification is possible, that higher degree of justification is made a requirement for knowledge.

The strength of the justification you need in order to have knowledge depends on the object of knowledge - i.e. the thing you are trying to know.

Another problem with defining knowledge is known as the "Gettier problem". The Gettier problem arises when we give certain kinds of counterexamples to the JTB (justified true belief) definition. A counterexample is a case where the definition applies, but the word defined doesn't; or a case where the word defined applies, but the definition doesn't. Gettier counterexamples are examples where the definition, justified, true belief applies; but one nevertheless still doesn't have knowledge, so the word "knowledge" doesn't apply in that case. So let me give one such counterexample. This sort of counterexample is due to the philosopher Edmund Gettier:

Say there's a man you know named Jones, and you find out that Jones is going to be offered a job (the boss tells you this). So you're walking around somewhere and you see Jones, who for some reason is emptying out his pockets and counting out his change. He says that he has ten coins in his pocket. So now you have two justified beliefs: that Jones is going to get the job, and that Jones has ten coins. And so you infer from these two beliefs: The person who is going to get the job has ten coins. And that's a justified belief too, right? Because you're perfectly justified in believing that Jones is getting the job and that he has ten coins. So the person who is going to get the job has ten coins. Fine.

But now suppose that you applied for the job; and contrary to what you were told, it turns out you are going to get the job, not Jones. The boss only told you that Jones was going to get the job, so that he could surprise you. So it turns out, even though you originally had a justified belief that Jones was going to get the job, he didn't get it. And that happens sometimes: sometimes things that we're well within our rights to believe turn out, surprisingly, to be false. But now just on a lark you decide to empty out your pockets and lo and behold, you count out ten coins. So! It turns out that the person who is going to get the job does have ten coins.

Now think back, to before you knew you were going to get the job. You thought Jones was going to get it. And you believed this justifiedly, even though it turned out to be wrong. And you were also justified in believing that Jones has 10 coins. What follows? The person who is going to get the job has ten coins; and so you believed this justifiedly too. But it turns out that this was true. So you had a justified, true belief that the person who is going to get the job has ten coins. But clearly you didn't know that then. You thought it was Jones who was going to get the job and you based your claim on that false, but justified assumption. Nonetheless, what you inferred from that assumption was true! So you had a justified true belief; but you didn't have knowledge. Well, Gettier and a lot of other philosophers said, that means that knowledge must be something more than justified, true belief.

Externalist responses

Gettier's article was published in 1963. Right after that, for a good decade or more, there was an enormous number of articles trying to supply the missing fourth condition of knowledge. The big project was to try to figure out the "X" in the equation, Knowledge = belief + truth + justification + X. Whenever someone proposed an answer, someone else would come up with a new counterexample to shoot down that definition.

Some of the proposed solutions involve factors external to the agent. These responses are therefore called externalism. For example, one externalist response to the Gettier problem is to say that the justified, true belief must be caused (in the right sort of way) by the relevant facts.


When scientists or philosophers ask "Is knowledge possible?", they mean to say "Am I ever sufficiently justified in believing something in order to have knowledge?" Adherents of Philosophical skepticism often say "no". Philosopical skepticism is the position which critically examines whether the knowledge and perceptions people have is true; adherents of this position hold that one can never obtain true knowledge, since justification is never certain. This is a different position from Scientific skepticism, which is the practical stance that one should not accept the veracity of claims until solid evidence is produced.

See also: Epistemology -- Truth -- Wisdom

External links


  • Creath, Richard, "Induction and the Gettier Problem", Philosophy and Phenomenological Research, Vol.LII, No.2, June 1992.
  • Feldman, Richard, "An Alleged Defect in Gettier Counterexamples", Australasian Journal of Philosophy, 52 (1974): 68-69.
  • Gettier, Edmund, "Is Justified True Belief Knowledge?", Analysis 23 (1963): 121-23.
  • Goldman, Alvin I., "Discrimination and Perceptual Knowledge", Journal of Philosophy, 73.20 (1976), 771-791.
  • Hetherington, Stephen, "Actually Knowing", The Philosophical Quarterly, Vol.48, No. 193, October 1998.
  • Lehrer, Keith and Thomas D. Paxon, Jr., "Knowledge: Undefeated Justified True Belief", The Journal of Philosophy, 66.8 (1969), 225-237.
  • Levi, Don S., "The Gettier Problem and the Parable of the Ten Coins", Philosophy, 70, 1995.
  • Swain, Marshall, "Epistemic Defeasibility", American Philosophical Quarterly, Vol.II, No.I, January 1974.

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