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In logic, an argument is valid if and only if its conclusion is logically entailed by its premises. A formula is valid if and only if it is true under every interpretation, and an argument form (or schema) is valid if and only if every argument of that logical form is valid.
An argument is valid if and only if the truth of its premises entails the truth of its conclusion and each step, sub-argument, or logical operation in the argument is valid. Under such conditions it would be self-contradictory to affirm the premises and deny the conclusion. The corresponding conditional of a valid argument is a logical truth and the negation of its corresponding conditional is a contradiction. The conclusion is a logical consequence of its premises.
An argument that is not valid is said to be "invalid".
An example of a valid argument is given by the following well-known syllogism:
What makes this a valid argument is not that it has true premises and a true conclusion, but the logical necessity of the conclusion, given the two premises. The argument would be just as valid were the premises and conclusion false. The following argument is of the same logical form but with false premises and a false conclusion, and it is equally valid:
No matter how the universe might be constructed, it could never be the case that these arguments should turn out to have simultaneously true premises but a false conclusion. The above arguments may be contrasted with the following invalid one:
In this case, the conclusion contradicts the deductive logic of the preceding premises, rather than deriving from it. Therefore the argument is logically 'invalid', even though the conclusion could be considered 'true' in general terms. The premise 'All men are immortal' would likewise be deemed false outside of the framework of classical logic. However, within that system 'true' and 'false' essentially function more like mathematical states such as binary 1s and 0s than the philosophical concepts normally associated with those terms.
A standard view is that whether an argument is valid is a matter of the argument's logical form. Many techniques are employed by logicians to represent an argument's logical form. A simple example, applied to two of the above illustrations, is the following: Let the letters 'P', 'Q', and 'S' stand, respectively, for the set of men, the set of mortals, and Socrates. Using these symbols, the first argument may be abbreviated as:
Similarly, the third argument becomes:
An argument is termed formally valid if it has structural self-consistency, such as when the operands between premises are all true, and the derived conclusion is also true. In the third example, the initial premises cannot logically result in the conclusion and is therefore categorized as an invalid argument, while the same statement in most contexts would be considered 'valid' unless Socrates was in fact immortal.
Note that the classically qualified 'strength' of arguments is defined separately from their structural validity. For example, the previous arguments would be formally called 'strong' ones, regardless of their validity. However, they differ from this version of the Socrates mortality example:
In formal logic terms this is considered an extremely 'weak' argument, however it can not be categorized as structurally invalid. While the premises certainly don't definitively lead to the conclusion, they do not logically preclude it. Strong and weak descriptors of arguments are generally related to inductive approaches to logic, as opposed to the more absolute qualifications of deductive methods. 
A statement can be called valid, i.e. logical truth, if it is true in all interpretations.
Validity of deduction is not affected by the truth of the premise or the truth of the conclusion. The following deduction is perfectly valid:
The problem with the argument is that it is not sound. In order for a deductive argument to be sound, the deduction must be valid and all the premises true.
Model theory analyzes formulae with respect to particular classes of interpretation in suitable mathematical structures. On this reading, formula is valid if all such interpretations make it true. An inference is valid if all interpretations that validate the premises validate the conclusion. This is known as semantic validity.
In truth-preserving validity, the interpretation under which all variables are assigned a truth value of 'true' produces a truth value of 'true'.
In a false-preserving validity, the interpretation under which all variables are assigned a truth value of 'false' produces a truth value of 'false'.
|Preservation properties||Logical connective sentences|
|True and false preserving:||Logical conjunction (AND, ) • Logical disjunction (OR, )|
|True preserving only:||Tautology ( ) • Biconditional (XNOR, ) • Implication ( ) • Converse implication ( )|
|False preserving only:||Contradiction ( ) • Exclusive disjunction (XOR, ) • Nonimplication ( ) • Converse nonimplication ( )|
|Non-preserving:||Proposition • Negation ( ) • Alternative denial (NAND, ) • Joint denial (NOR, )|
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|Definitions and translations from Wiktionary|