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TruthWhen someone sincerely agrees with an assertion, they are claiming that it is the truth. Epistemology, the study of knowledge, seeks solutions for the many philosophicalproblems associated with truth.
Philosophical problemsThe 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 truthPhilosophers call any entity that can be true or false a "truth bearer." Propositions, sentences, statements, ideas, beliefs, and judgementsare 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 parts of 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 truthPhilosophersand logicianshave proposed a number of broad theoriesabout truth, which are now frequently sorted into two camps: Robust theoriesSome 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 "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:
Deflationary theoriesOther 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 infinitely long sentences. For example, I cannot express my confidence in Michael's accuracy by asserting the endless sentence:
But I can express it succinctly just by saying:
Once we have identified the truth predicate's formal features and utility, deflationists 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 Fregeand 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 truthholds 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 actof 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 Belnapas 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. Formal definitionsSemantic theory of truthThe semantic theory of truthhas as its general case for a given language:
where 'P' is a reference to the sentence (the sentence's name), and P is just the sentence itself. Logician and philosopher Alfred Tarskideveloped 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 Davidsonused it as the foundation of his truth-conditional semanticsand linked it to radical interpretationin a form of coherentism. Kripke's theory of truthSaul Kripkecontends 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:
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 truthSubjective versus objectiveSubjective 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 subjectivismholds 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, objectivetruths 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 absoluteRelative 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 Relativismis 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 relativismis 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 demanddetermine 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. Absolutismin 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 absolutismis 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:
Double truthIn thirteenth century Europe, the Roman Catholic Churchdenounced 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 CatholicChurch was based upon neoplatonicideas, 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. True testimonyWitnesseswho swearunder oathto testifytruthfully in courtsof law, are not expected to make infallibly true statements, but to make a good faithattempt to recount an observed eventfrom their memoryor 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. See alsoImage:Wikiquote-logo-en.png
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Truth in logic
Major philosophers who have proposed theories of truth
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