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When someone sincerely agrees with an assertion, they are claiming that it is the truth. Epistemology, the study of knowledge, seeks solutions for the many philosophical problems associated with truth, as well as ways to convince other people that this is an important pursuit that deserves funding.
The first problem for philosophers is deciding what sorts of things are true or false, the so-called truth-bearers. At stake is the terminology we use to discuss truth. Then there are a range of theories about what makes these truth-bearers true. Some, the robust theories, treat truth as a property; others, the deflationary theories, suggest that it is no more than a convenient tool in our language. Developments in formal logic have thrown light on the way in which truth is used both in formal systems and in natural languages.
Standing beside these problems are the issues of how we know something to be true. The way in which one knows that one has a toothache seems different from the way in which one knows that the Earth is the third planet from the sun; perhaps one is subjective, and determined by introspection, while the other objective, and determined by a combination of shared observations, reasonings, and calculations. Similarly, some truths seem to be relative to one's position or background, while others appear absolute. Philosophers have diverse opinions on each of these issues.
Bearers of truth
Philosophers call any entity that can be true or false a "truth bearer." Propositions, sentences, statements, ideas, beliefs, and judgements are said to be truth bearers. Thus, a truth bearer, in the philosophical sense, is not a person or god.
Some philosophers exclude one or more of these categories, or argue that some of them are true (or false) only in a derivative sense. These claims are made on the basis of theories about truth such as those discussed below.
For example, propositions are often thought to be the only things that are literally true. A proposition is the abstract entity which is expressed by a sentence, held in a belief, affirmed in a statement or judgement. All these things (which are parts of a language) are called true only if they express, hold, or affirm true propositions. So plausibly sentences of different languages, such as the (English) The sky is blue and the (German) Der Himmel ist blau express the same proposition.
On the other hand, many philosophers have claimed that propositions and similar abstract entities are mysterious and provide little explanation; surely sentences, or even utterances of sentences, are a more clear-cut and fundamental truth bearer.
Theories about truth
Some theories hold in common that truth is a robust (sometimes inflationary) concept. These theories all hold that the surface grammar of sentences that seem to predicate truth or falsity, such as "Snow is white is true" can be taken at face value. Truth is a property, just as red is a property predicated of a barn in the sentence in "The barn is red." The task for such theories is to explain the nature of this property. Hence, according to these theories, truth needs explanation and is something about which significant things can be said:
- The correspondence theory of truth sees truth as correspondence with objective reality. Thus, a sentence is said to be true just in the case that it expresses a state of affairs in the world.
- The coherence theory sees truth as coherence with some specified set of sentences or, more often, of beliefs. For example, one of a person's beliefs is true just in case it is coherent with all or most of her other beliefs. Usually, coherence is taken to imply something stronger than mere consistency: justification, evidence, and comprehensiveness of the belief set are common restrictions.
- The consensus theory holds that truth is whatever is agreed upon, or in some versions, might come to be agreed upon, by some specified group.
- Pragmatism sees truth as the success of the practical consequences of an idea, i.e. its utility.
- Social constructivism holds that truth is constructed by social processes, is historically and culturally specific, and that it is in part shaped through the power struggles within a community.
- The Indefinability theory of truth views the concept of truth along the same lines as a correspondence theorist, but it holds that truth cannot be defined in terms of simpler concepts.
Other philosophers reject the idea that truth is a robust concept in this sense. From this point of view, to say "2 + 2 = 4 is true" is to say no more than that "2 + 2 = 4", and that there is no more to say about truth than this. These positions are broadly called "deflationary" theories of truth (because the concept has been "deflated" of importance) or "disquotational" theories (to draw attention to the mere "disappearance" of the quotation marks in cases like the above example).
In addition to highlighting this formal feature of the predicate "is true", some deflationists point out that the concept enables us to express things that might otherwise require an infinitely long sentences. For example, I cannot express my confidence in Michael's accuracy by asserting the endless sentence:
- Michael says, 'snow is white' and snow is white, or he says 'roses are red' and roses are red or he says ... etc.
But I can express it succinctly just by saying:
- Whatever Michael says is true.
Once we have identified the truth predicate's formal features and utility, deflationsists argue, we have said all there is to be said about truth. The primary theoretical concern of these views is to explain away those special cases where it appears that the concept of truth does have peculiar and interesting properties. (See Semantic paradoxes, and below.)
From this point of view (see Gottlob Frege and F. P. Ramsey), truth is not the name of some property of propositions — some thing about which one could have a theory. The belief that truth is a property is just an illusion caused by the fact that we have the predicate "is true" in our language. Since most predicates name properties, we naturally assume that "is true" does as well. But, deflationists say, statements that seem to predicate truth actually do nothing more than signal agreement with the statement.
For example, the redundancy theory of truth holds that to assert that a statement is true is just to assert the statement itself. Thus, to say that "Snow is white" is true is to say nothing more nor less than that snow is white.
A second example, attributed to P. F. Strawson, is the performative theory of truth which holds that to say "Snow is white" is true is to perform the speech act of signalling one's agreement with the claim that snow is white (much like nodding one's head in agreement). The idea that some statements are more actions than communicative statements is not as odd as it may seem. Consider, for example, that when the bride says "I do" at the appropriate time in a wedding, she is performing the act of taking this man to be her lawful wedded husband. She is not describing herself as taking this man.
A third type of deflationary theory is the disquotational theory which uses a variant form of Tarski's schema: To say that '"P" is true' is to say that P. One of the most thoroughly worked out versions of this view is the prosentential theory of truth, first developed by Dorothy Grover, Joseph Camp, and Nuel Belnap as an elaboration of Frank P. Ramsey's claims. They argue that sentences like "That's true" are prosentences (see pro-form), expressions that merely repeat the content of other expressions. In the same way that it means the same as my dog in the sentence My dog was hungry, so I fed it, That's true is supposed to mean the same as It's raining if you say the latter and I then say the former.
Semantic theory of truth
The semantic theory of truth has as its general case for a given language:
- 'P' is true if and only if P
where 'P' is a reference to the sentence (the sentence's name), and P is just the sentence itself.
Logician and philosopher Alfred Tarski developed the theory for formal languages (such as formal logic). Here he restricted it in this way: no language could contain its own truth predicate, that is, the expression is true could only apply to sentences in some other language. The latter he called an object language, the language being talked about. (It may, in turn, have a truth predicate that can be applied to sentences in still another language.) The reason for his restriction was that languages that contain their own truth predicate will contain paradoxical sentences like the Liar: This sentence is not true. See The Liar Paradox. As a result Tarski held that the semantic theory could not be applied to any natural language, such as English, because they contain their own truth predicates. Tarski thought of his theory as a species of correspondence theory. Donald Davidson used it as the foundation of his truth-conditional semantics and linked it to radical interpretation in a form of coherentism.
Kripke's theory of truth
Saul Kripke contends that a natural language can in fact contain its own truth predicate without giving rise to contradiction. He showed how to construct one as follows:
- Begin with a subset of sentences of a natural language that contains no occurrences of the expression "is true" (or "is false"). So The barn is big is included in the subset, but not ' The barn is big is true', nor problematic sentences such as "This sentence is false".
- Define truth just for the sentences in that subset.
- Then extend the definition of truth to include sentences that predicate truth or falsity of one of the original subset of sentences. So ' The barn is big is true' is now included, but not either This sentence is false nor "' The barn is big is true' is true".
- Next, define truth for all sentences that predicate truth or falsity of a member of the second set. Imagine this process repeated infinitely, so that truth is defined for The barn is big; then for ' The barn is big is true'; then for "' The barn is big is true' is true", and so on.
Notice that truth never gets defined for sentences like This sentence is false, since it was not in the original subset and does not predicate truth of any sentence in the original or any subsequent set. In Kripke's terms, these are "ungrounded." Since these sentences are never assigned either truth or falsehood even if the process is carried out infinitely, Kripke's theory implies that some sentences are neither true nor false. This contradicts the Principle of bivalence: every sentence must be either true or false. Since this principle is a key premise in deriving the Liar paradox, the paradox is dissolved.
Types of truth
Subjective versus objective
Subjective truths are those with which we are most intimately acquainted. That I like broccoli or that I have a pain in my foot are both subjectively true. Metaphysical subjectivism holds that all we have are such truths. That is, that all we can know about are, one way or another, our own subjective experiences. This view does not necessarily reject realism. But at the least it claims that we cannot have direct knowledge of the real world.
In contrast, objective truths are supposed in some way to be independent of our subjective beliefs and tastes. Such truths would subsist not in the mind but in the external object.
Relative versus absolute
Relative truths are statements or propositions that are true only relative to some standard or convention or point-of-view. Usually the standard cited is the tenets of one's own culture. Everyone agrees that the truth or falsity of some statements is relative: That the fork is to the left of the spoon depends on where one stands. But Relativism is the doctrine that all truths within a particular domain (say, morality or aesthetics) are of this form, and Relativism entails that what is true varies across cultures and eras. For example, Moral relativism is the view that moral truths are socially determined. Some logical issues about Relativism are taken up in the article on the relativist fallacy.
Relative truths can be contrasted with absolute truths. The latter are statements or propositions that are taken to be true for all cultures and all eras. For example, for Muslims "God is great" expresses an absolute truth; for the microeconomist, that the laws of supply and demand determine the value of any consumable in a market economy is true in all situations; for the Kantian, "act only according to that maxim by which you can at the same time will that it should become a universal law" forms an absolute moral truth. They are statements that are often claimed to emanate from the very nature of the universe, God, human nature, or some other ultimate essence or transcendental signifier.
Absolutism in a particular domain of thought is the view that all statements in that domain are either absolutely true or absolutely false: none is true for some cultures or eras while false for other cultures or eras. For example, Moral absolutism is the view that moral claims such as "Abortion is wrong" or "Charity is good" are either true for all people in all times or false for all people in all times.
Other uses of "Truth"
In addition to its use in reference to propositions, there are other uses of "truth" and "true" in the English language:
- most often applied to people, and is used as a commendation, synonymous with "loyal", as in she is true to her friends. This sense of truth should be contrasted with being fake, insincere, misleading and so on.
- True can mean "in accordance with a standard or archetype," which is how it is used in "He is a true Englishman."
- True in engineering and construction can be used as meaning "straight", not warped but in the same flat plane - as the spokes of a wheel.
In thirteenth century Europe, the Roman Catholic Church denounced what it described as theories of "double truth," i.e. theories to the effect that although a truth may be established by reason, its contrary ought to be believed as true as a matter of faith.
The condemnation was aimed specifically at a "Latin Averroist," (see Averroës), Siger of Brabant, but it was more broadly an attempt to halt the spread of Aristotle's ideas, which the reconquest of Spain and, accordingly, access to the libraries of the Moors had re-introduced into the Latin literate world. At the time, much of the doctrine of the Roman Catholic Church was based upon neoplatonic ideas, and Aristoteleanism struck many as heresy. Siger and others seem to have conceded this, and to have used the sharp reason/faith distinction that came to be known as "double truth" as a way of legitimizing discussion of Aristotle despite that concession.
Witnesses who swear under oath to testify truthfully in courts of law, are not expected to make infallibly true statements, but to make a good faith attempt to recount an observed event from their memory or provide expert testimony. That what one witness says may differ from true accounts of other witnesses is a commonplace occurrence in the practice of law. Triers-of-fact are then charged with the responsibility to determine the credibility or veracity of a witness's testimony.
- Epistemic theories of truth
- Liar paradox
- Philalethia (love of truth)
- Unity of the proposition