“”Yes, there is a Divinity, one from which we must never turn aside for the guidance of our huge inward life and of the share we have as well in the life of all men. It is called the truth.
|—Henri Barbusse in Under Fire|
Truth is a concept that is easy to recognize by common sense but extremely difficult to actually define. It is the subject of great controversy in philosophy, specifically because "it is true that a certain definition of truth is accurate" is a self-referential statement.
The common-sense understanding of "truth" is captured by this quote from Aristotle: "If I say of what is that it is, I speak the truth." Thus, in this sense, truth bears a close correspondence to reality (although many philosophers differ, as is their wont).
Truth is largely independent of ideology; but partisans seem to be more reluctant to speak it when it conflicts with their ideology, and also speak it with much more enthusiasm if it supports their ideology.
Ideas which work consistently can be used to make correct predictions, and we say they are true. The truth of an assertion does not rest on the asserter's goodness, but it is incumbent upon the asserter to make his case, not the skeptic to falsify the case. If the asserter tries to use his desired outcome as a premise, he hasn't made his case.
The individual prestige or infamy of a given person who believed a given thing doesn't have any effect on its truth or falsity. It is not valid to plead, "Einstein was a genius and he believed in God" as an argument for the existence of some god.
 Philosophical theories of truth
- Correspondence theory: A true statement is one that is in accordance with reality. This tradition follows Aristotle's definition, is still the most widely accepted by far, and an important epistemic foundation of the scientific method.
- Coherence theory: A statement is considered true if it is logically consistent with an already established system of other statements. Another, stricter version of this theory only accepts statements as true that are logically deduced from (or "entailed" by) an existing set of propositions. This approach is most prominently employed in mathematics, and used to be favored by some 20th century philosophers of science.
- Constructivist theory: Truth itself is seen as a social construct, and therefore contingent upon tradition, convention and perception. This approach is usually favored by relativists who like to deny that there could possibly be any "preferred" system or method of deriving objective facts. Moderate constructivists hold that while objective knowledge is possible and there most likely is a shared reality, individually diverging perceptions are of greater importance for how people interact with each other and the world.
- Consensus theory: The truth is whatever is agreed upon, or at least whatever would be agreed upon as the result of a free, rational discourse. This variant was strongly promoted by Jürgen Habermas in the 1970s, who seems to have given up on it nowadays. Except for sounding like a certain fallacy, the group of people who are allowed to settle the debate over some issues would necessarily have to be restricted to a much narrower one, since there are only few people who are informed enough about complex topics to participate in such discourse. The consensus theory does not provide a satisfying solution for situations in which expert opinion (a.k.a. "Scientific consensus") has reached a consensus, but one that is different from opinions within society at large.
- Pragmatic theory: This variant was introduced by the American pragmatist school, and most notably held by Charles Peirce and William James. In a nutshell, truth is whatever works - if belief in a certain proposition has beneficial consequences, it can be considered true. "Beneficial consequences" are usually defined as anything that allows humans to navigate the world successfully - so for example, belief in one's ability to fly is false since it is harmful. This approach also shares common ground with the scientific method: Dewey's update on this theory consisted of defining "truth" as the end product of the process of scientific enquiry. For pragmatists, it does not matter all that much whether a highly abstract theory (like much of modern physics) actually represents an accurate picture of reality or whether it is just made up of useful assumptions and analogies, as long as its predictions are accurate.
- Deflationary theory: Deflationists reject the idea that the word "truth" refers to a substantive property held by statements. They hold that there is no difference between the statements "Snow is white" and "It is true that snow is white," since they both hold if and only if snow is actually white. Other properties attributed to statements (such as peculiarity; e.g. "It is surprising that snow is white") do not have this property, which suggests it is not a property at all. Deflationists generally do not deny that there are statements which actually describe the world, but hold that assigning the word "true" to them is equivalent to asserting them.
- Disquotationalism: a position within deflationism which explains the nature of some statements attributing truth as functioning grammatically, not semantically. The sentence "Bob's statement is true" is interpreted by disquotationalists not as assigning the property "truth" to Bob's statement but simply as shorthand for accepting it; the word "true" is understood to be useful in that it allows us to reaffirm Bob's statement without repeating it word-for-word, in a grammatically acceptable manner.
 Role in logic
Truth is a fundamental concept in logic, the branch of philosophy that deals with the formal correctness of arguments. Each sentence or construct in logic is usually assigned a "truth value" that represents, in various ways, whether the construct is "true." Arguments are a method of propagating truth values through the logical system; for example, in propositional logic, the modus ponens inference rule says that if the sentences "if A then B" and "A" are given the value of "true," then the sentence "B" must also be given the value of "true."
Different systems of logic are often classed according to the range of truth values a sentence can be assigned. Among the most popular are:
- Binary logic: Each statement is either true (1) or false (0). Binary logics include the classical systems, propositional and first-order.
- Ternary logic: A statement can have one of three values, usually true, false, or unknown/irrelevant.
- Fuzzy logic: Truth values are continuous rather than discrete, being assigned a real number between 0 and 1. Such a truth value is not the same as a probability, which instead assigns a likelihood that a statement is 100% true; while probability says that "A has a 50% chance of being completely true," fuzzy logic says that "A is 50% true."
Logic is strictly a system of propagation for truth, working from assumptions that certain premises or axioms are true. Hence, in logic, the concept of truth is ignored and left to other branches of philosophy; the concept stays an empty shell, so to speak.