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In logic, specifically in deductive reasoning, an argument is valid if and only if it takes a form that makes it impossible for the premises to be true and the conclusion nevertheless to be false.[1] It is not required for a valid argument to have premises that are actually true,[2] but to have premises that, if they were true, would guarantee the truth of the argument's conclusion. Valid arguments must be clearly expressed by means of sentences called well-formed formulas (also called wffs or simply formulas).

The validity of an argument can be tested, proved or disproved, and depends on its logical form.[3]

Arguments

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Argument terminology used in logic

In logic, an argument is a set of related statements expressing the premises (which may consists of non-empirical evidence, empirical evidence or may contain some axiomatic truths) and a necessary conclusion based on the relationship of the premises.

An argument is valid if and only if it would be contradictory for the conclusion to be false if all of the premises are true.[3] Validity does not require the truth of the premises, instead it merely necessitates that conclusion follows from the premises without violating the correctness of the logical form. If also the premises of a valid argument are proven true, this is said to be sound.[3]

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 necessary consequence of its premises.

An argument that is not valid is said to be "invalid".

An example of a valid (and sound) argument is given by the following well-known syllogism:

All men are mortal. (True)
Socrates is a man. (True)
Therefore, Socrates is mortal. (True)

What makes this a valid argument is not that it has true premises and a true conclusion. Validity is about the tie in relationship between the two premises the necessity of the conclusion. There needs to be a relationship established between the premises i.e., a middle term between the premises. If you just have two unrelated premises there is no argument. Notice some of the terms repeat: men is a variation man in premises one and two, Socrates and the term mortal repeats in the conclusion. The argument would be just as valid if both premises and conclusion were false. The following argument is of the same logical form but with false premises and a false conclusion, and it is equally valid:

All cups are green. (False)
Socrates is a cup. (False)
Therefore, Socrates is green. (False)

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:

All men are immortal. (False)
Socrates is a man. (True)
Therefore, Socrates is mortal. (True)

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. Formal arguments that are invalid are often associated with at least one fallacy which should be verifiable.

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:

All P are Q.
S is a P.
Therefore, S is a Q.

Similarly, the third argument becomes:

All P's are not Q.
S is a P.
Therefore, S is a Q.

An argument is termed formally valid if it has structural self-consistency, i.e. if when the operands between premises are all true, the derived conclusion is always also true. In the third example, the initial premises cannot logically result in the conclusion and is therefore categorized as an invalid argument.

Valid formula

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A formula of a formal language is a valid formula if and only if it is true under every possible interpretation of the language. In propositional logic, they are tautologies.

Statements

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A statement can be called valid, i.e. logical truth, in some systems of logic like in Modal logic if the statement is true in all interpretations. In Aristotelian logic statements are not valid per se. Validity refers to entire arguments. The same is true in propositional logic (statements can be true or false but not called valid or invalid).

Soundness

[edit]
Main article: Soundness

Validity of deduction is not affected by the truth of the premise or the truth of the conclusion. The following deduction is perfectly valid:

All animals live on Mars. (False)
All humans are animals. (True)
Therefore, all humans live on Mars. (False)

The problem with the argument is that it is not sound. In order for a deductive argument to be sound, the argument must be valid and all the premises must be true.[3]

Satisfiability

[edit]
Main article: Satisfiability

Model theory analyzes formulae with respect to particular classes of interpretation in suitable mathematical structures. On this reading, a 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.[4]

Preservation

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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'.[5]

Preservation properties Logical connective sentences
True and false preserving: Proposition  • 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: Negation ( )  • Alternative denial (NAND, ) • Joint denial (NOR, )

See also

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References

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Further reading

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Validity (logic)

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In logic, validity is a fundamental property of deductive arguments, defined as the condition under which it is impossible for all premises to be true while the conclusion is false, thereby ensuring that the truth of the premises guarantees the truth of the conclusion.[1] This concept emphasizes the structural integrity of reasoning rather than the actual truth values of the statements involved, making it essential for assessing the reliability of inferences in fields such as philosophy, mathematics, and computer science.[2] The origins of validity in logical reasoning date back to ancient Greece, where Aristotle (384–322 BCE) pioneered deductive logic through his development of the syllogism—a structured argument with two premises and a conclusion that follows necessarily if the form is correct.[3] In works like the Prior Analytics, Aristotle outlined specific figures and moods of syllogisms that qualify as valid, establishing a systematic approach to deduction that influenced Western thought for over two millennia.[3] For instance, the classic syllogism "All humans are mortal; Socrates is human; therefore, Socrates is mortal" exemplifies validity through its categorical structure, where the conclusion is inescapably true if the premises hold. In modern formal logic, validity has been rigorously formalized across systems like propositional and predicate logic. Semantically, a formula or argument is valid if it holds true under every possible interpretation or model, such as all truth assignments for propositional variables.[4] For example, the propositional formula
(p→q)→(¬q→¬p) (p \to q) \to (\neg q \to \neg p)
is valid because it is a tautology, true regardless of the truth values assigned to $ p $ and $ q $.[5] Syntactically, validity can also be proven through derivation rules in proof systems, confirming that the conclusion is a logical consequence of the premises without empirical verification. Validity is distinct from soundness, which requires not only a valid structure but also true premises, thereby guaranteeing a true conclusion.[1] An argument can be valid yet unsound if its premises are false, as in "All ducks are mammals; Daffy Duck is a duck; therefore, Daffy Duck is a mammal," where the second premise is false despite the logical form being impeccable.[2] This distinction underscores validity's role in preserving truth transmission, while soundness evaluates overall argumentative strength. In contemporary applications, validity testing via truth tables or model checking remains crucial for automated reasoning and artificial intelligence.[6]

Core Concepts

Definition

In logic, validity is a fundamental property of arguments and inferences, characterized semantically as follows: an argument is valid if and only if, in every possible interpretation or model, whenever all of its premises are true, its conclusion is also true—or equivalently, there is no interpretation in which the premises are true and the conclusion false.[7] This definition captures the idea of necessary implication, ensuring that the structure of the argument preserves truth from premises to conclusion across all conceivable scenarios.[8] The concept of validity originates in ancient Greek philosophy, particularly in the works of Aristotle, who developed syllogistic logic as a system for evaluating deductive inferences based on categorical propositions, where a syllogism is valid if the truth of its premises necessitates the truth of its conclusion.[9] This informal notion was later formalized in modern terms by Alfred Tarski in his 1936 essay "On the Concept of Logical Consequence," which provided a precise semantic account of logical consequence, distinguishing it from syntactic or proof-based approaches and establishing validity as a model-theoretic property.[8] A classic example of a valid argument is the Aristotelian syllogism: "All men are mortal; Socrates is a man; therefore, Socrates is mortal." In every interpretation where the premises hold true—meaning the class of men is a subset of mortals, and Socrates belongs to the class of men—the conclusion must also be true, as it follows inescapably from the premises.[9] Validity must be distinguished from truth: while truth applies to individual propositions or sentences (being true or false in a given interpretation), validity pertains solely to the relational structure of arguments, assessing whether the conclusion is necessarily implied by the premises regardless of their actual truth values in the world.[7] Thus, a valid argument may contain false premises yet still qualify as valid, whereas an argument with true premises and a true conclusion requires validity for soundness.[8]

Truth Preservation

In logic, the principle of truth preservation captures the essence of validity by ensuring that the truth of an argument's premises guarantees the truth of its conclusion without exception. Formally, an argument is valid if, in every model or interpretation where all premises hold true, the conclusion also holds true in that same model. This semantic condition, originally articulated by Alfred Tarski in his foundational work on truth and logical consequence, defines validity as a relation of entailment where the conclusion is a logical consequence of the premises. This preservation operates independently of whether the premises are actually true in a given context; validity is assessed hypothetically across all possible scenarios. Thus, even if the premises are false, the argument remains valid provided no counterexample exists—an interpretation where the premises are true but the conclusion is false. Such counterexamples serve to invalidate arguments by demonstrating a failure in truth transmission, as emphasized in standard treatments of deductive logic.[10] A classic illustration of truth preservation is the inference rule of modus ponens, which takes the form: from "If P then Q" and "P," conclude "Q." Assuming the premises are true forces the conclusion to be true, as the conditional links the antecedent to the consequent, preserving truth across all relevant interpretations without allowing a scenario where the premises hold but the conclusion fails. This exemplifies how validity ensures reliable inference in deductive arguments.[11] Ultimately, validity equates to the necessary preservation of truth under every possible interpretation, reflecting a modal necessity in logical structure.

Validity in Arguments

Deductive Arguments

Deductive arguments form the primary framework for assessing logical validity, consisting of one or more premises—statements assumed to be true for the sake of argument—a conclusion that is the claim inferred from those premises, and inference rules that systematically link the premises to the conclusion to ensure the inference's necessity.[12] These components allow for the evaluation of whether the conclusion follows inescapably from the premises, independent of their actual truth values.[13] Unlike inductive arguments, which offer probabilistic support for a conclusion and are assessed for strength rather than validity, deductive arguments seek absolute certainty: if the premises hold, the conclusion must hold without exception.[14] This emphasis on necessary inference distinguishes deductive reasoning as the core domain where validity is defined and tested. The historical roots of deductive arguments trace to Aristotle's syllogistic logic in the 4th century BCE, where he formalized deductive reasoning through categorical syllogisms in works like the Prior Analytics, identifying valid forms based on term relations and propositional structures.[15] During medieval scholasticism (5th–15th centuries), this foundation evolved through refinements by figures such as Boethius, who integrated Stoic elements like conditionals; Abelard, who advanced propositional connectives and negation; and William of Ockham, who developed rules for consequences (consequentiae) and supposition theory to handle term ambiguities in arguments.[15] The tradition culminated in modern symbolic logic in the 19th and 20th centuries, with George Boole's algebraic representation of classes and operations, Gottlob Frege's introduction of quantifiers and predicate calculus in Begriffsschrift (1879), and Bertrand Russell and Alfred North Whitehead's axiomatic system in Principia Mathematica (1910–1913), shifting deductive arguments toward abstract, mathematical precision.[15][16] In general form, a deductive argument schema presents a finite sequence of statements, with initial premises followed by intermediate steps via inference rules, culminating in the conclusion that necessarily follows, thereby preserving truth across the inference.[15]

Valid Arguments

In deductive logic, a valid argument is one where the truth of the premises guarantees the truth of the conclusion, meaning there is no possible situation or interpretation in which the premises are true while the conclusion is false.[17] This criterion ensures that the argument's structure preserves truth from premises to conclusion, distinguishing validity as a formal property independent of the actual truth values of the statements involved.[1] Common examples of valid argument forms include modus tollens and disjunctive syllogism. Modus tollens takes the structure: If P, then Q; not Q; therefore, not P—for instance, "If it rains, the ground gets wet; the ground is not wet; therefore, it did not rain."[18] Disjunctive syllogism follows: P or Q; not P; therefore, Q—such as, "Either the team wins or it ties; the team did not win; therefore, it tied."[19] In contrast, an invalid form like affirming the consequent—"If P, then Q; P; therefore, Q," as in "If it rains, the ground gets wet; it rained; therefore, the ground is wet"—fails validity because the conclusion could be false even if the premises are true, such as when sprinklers wet the ground instead.[17] To test for validity, informal methods involve searching for counterexamples, where one attempts to construct a scenario making the premises true and the conclusion false; the absence of such a counterexample supports validity, while finding one proves invalidity.[20] Formal methods rely on constructing proofs using rules of inference, such as natural deduction, to derive the conclusion from the premises within a logical system, confirming validity if the derivation succeeds.[21] Validity extends to natural language arguments in deductive reasoning by translating them into symbolic logical forms, allowing assessment of their structural soundness despite ambiguities in everyday expression.[22]

Formal Systems

Valid Formulas

In formal logic, a well-formed formula (WFF) ϕ\phi is valid, denoted ⊨ϕ\models \phi, if it is true in every possible model or interpretation of the language.[7] This semantic notion captures the idea that ϕ\phi holds universally, regardless of how the non-logical symbols (such as predicate or propositional variables) are assigned meanings in a given structure. Validity thus serves as a cornerstone for evaluating the logical strength of formulas within deductive systems, distinguishing those that are necessarily true from those that may hold contingently.[23] Well-formed formulas in formal logic are built recursively from atomic components using syntactic rules. These include propositional variables (e.g., PP, QQ) or predicate symbols applied to terms (e.g., P(x)P(x)), combined via logical connectives such as conjunction (∧\land), disjunction (∨\lor), implication (→\to), and negation (¬\lnot), and optionally extended with quantifiers like universal (∀\forall) and existential (∃\exists) in predicate logics.[7] This syntax-semantics linkage ensures that the structural rules for formula construction align with truth conditions in interpretations, where models specify a domain of objects and interpretations for symbols to evaluate formula truth.[23] Representative examples of valid formulas include tautologies in propositional logic, such as P→PP \to P (reflexivity of implication) and P∨¬PP \lor \lnot P (law of excluded middle), which are true under all truth-value assignments to PP.[23] In first-order logic, universal validity extends this to quantified structures, as in ∀x(P(x)→P(x))\forall x (P(x) \to P(x)), which holds in every non-empty domain interpretation.[7] Semantic validity coincides with syntactic provability in complete formal systems. Gödel's completeness theorem (1930) establishes that for classical first-order logic, every valid formula is provable from the axioms using the inference rules, ensuring the deductive system captures all semantic truths.[7]

Semantic Validity

Semantic validity in logic is grounded in model theory, which provides a mathematical framework for interpreting formal languages and determining the truth of formulas within specific structures. An interpretation, often denoted as a model $ M $, consists of a non-empty set $ D $ called the domain, along with interpretations for the non-logical symbols of the language: constant symbols are assigned elements from $ D $, function symbols are assigned functions on $ D $, and predicate symbols are assigned relations on $ D $. The satisfaction relation $ M \models \phi $ [satisfaction] defines whether a formula $ \phi $ holds true in $ M $ under a given assignment of values to the free variables in $ \phi $, recursively specified for atomic formulas and extended to compound formulas using the connectives and quantifiers of the logic. This setup, pioneered by Alfred Tarski, ensures that truth is relativized to the model, allowing for precise evaluation of logical expressions across varying structures.[24] A formula $ \phi $ is semantically valid, denoted $ \models \phi $, if it is satisfied in every possible model $ M $ for the language. More generally, semantic entailment between a set of premises $ \Gamma $ and a conclusion $ \phi $, written $ \Gamma \models \phi $, holds if every model $ M $ that satisfies all formulas in $ \Gamma $ (i.e., $ M \models \psi $ for all $ \psi \in \Gamma $) also satisfies $ \phi $. This definition captures the idea that the truth of the premises necessitates the truth of the conclusion across all interpretations, providing a model-theoretic criterion for logical consequence independent of any proof system. Tarski formalized this notion in his work on the concept of logical consequence, emphasizing that it preserves truth in all models where the premises are true.[24] In classical first-order logic, semantic validity is intimately connected to syntactic provability through Gödel's completeness theorem, which asserts that a formula $ \phi $ is provable from a set of axioms and inference rules (denoted $ \Gamma \vdash \phi $) if and only if it is semantically entailed by $ \Gamma $ (denoted $ \Gamma \models \phi ).Thesoundnessdirection(). The soundness direction ( \Gamma \vdash \phi $ implies $ \Gamma \models \phi )ensuresthatprovableformulasareindeedtrueinallmodels,whilecompleteness() ensures that provable formulas are indeed true in all models, while completeness ( \Gamma \models \phi $ implies $ \Gamma \vdash \phi $) guarantees that every semantic consequence is capturable by formal proofs. This equivalence, established by Kurt Gödel in 1930, bridges the syntactic and semantic perspectives, confirming that the standard proof systems for classical logic are adequate for capturing all model-theoretic truths.[25] Semantic validity exhibits variations in non-classical logics, where the underlying models differ from classical ones to reflect alternative notions of truth. For instance, in intuitionistic logic, validity is assessed using Kripke models, which consist of a partially ordered set of worlds with a monotonic forcing relation for atomic predicates, ensuring that once a proposition is true at a world, it remains true at all accessible future worlds. In such models, the law of excluded middle ($ \phi \vee \neg \phi $) is not valid, as there exist models where neither $ \phi $ nor $ \neg \phi $ (defined as $ \neg \phi \equiv \phi \to \bot $, with $ \bot $ false at all worlds) holds at the initial world, though the models force classical tautologies that do not rely on excluded middle. This semantic framework, developed by Saul Kripke, highlights how intuitionistic validity prioritizes constructive proof over bivalent truth, distinguishing it from classical semantics.[26]

Related Concepts

Soundness

In deductive logic, an argument is sound if it is valid and all of its premises are actually true. This definition combines the structural necessity of validity—where the conclusion follows necessarily from the premises—with the factual accuracy of the premises in the real world. Soundness thus requires empirical verification of the premises beyond mere logical form.[2] The implications of soundness are significant: a sound argument guarantees a true conclusion, as the truth of the premises ensures the conclusion's truth through valid inference. Unsoundness occurs if either the argument lacks validity or any premise is false, potentially leading to erroneous conclusions despite correct structure. Validity serves as a necessary but insufficient condition for soundness, emphasizing that logical form alone does not suffice without true starting points.[27][28] A classic example of a sound argument is the syllogism: All men are mortal; Socrates is a man; therefore, Socrates is mortal. This is valid in form, and the premises are empirically true—all humans are mortal, and Socrates was human—rendering the argument sound and its conclusion undeniably true. In contrast, consider the valid but unsound argument: All ducks are mammals; Daffy Duck is a duck; therefore, Daffy Duck is a mammal. The form preserves truth, but the premise that all ducks are mammals is false, making the argument unsound.[27][2] Soundness bridges formal logic to epistemology by linking structural validity to knowledge claims, as it provides a reliable mechanism for deriving true conclusions from verified premises, thereby supporting justified beliefs about the world.[29]

Satisfiability

In logic, a formula ϕ\phi is satisfiable, denoted Sat(ϕ)\mathrm{Sat}(\phi), if there exists at least one model MM such that M⊨ϕM \models \phi; otherwise, it is unsatisfiable.[7] A set of formulas Γ\Gamma is satisfiable if there is a model MM satisfying every formula in Γ\Gamma. Unsatisfiability of ϕ\phi is equivalent to the validity of its negation, ⊨¬ϕ\models \neg \phi.[7] Satisfiability stands in duality to validity: a formula ϕ\phi is valid (⊨ϕ\models \phi) if and only if its negation ¬ϕ\neg \phi is unsatisfiable. A formula is contingent if it is satisfiable but not valid, meaning both ϕ\phi and ¬ϕ\neg \phi are satisfiable (i.e., ϕ\phi is refutable but not falsifiable in all models). This relation underscores the model-theoretic foundation of classical logic, where validity requires truth in all models and satisfiability requires truth in at least one.[23][7] For propositional logic, satisfiability is decidable via truth tables, a systematic enumeration of all 2n2^n possible truth-value assignments to nn atomic propositions, checking if any assignment makes the formula true. This provides a complete decision procedure, as the finite nature of propositional interpretations ensures termination. In contrast, satisfiability for full first-order logic is undecidable, meaning no general algorithm exists to determine it for arbitrary formulas; this was independently proven by Alonzo Church and Alan Turing in 1936 through reductions to undecidable problems in lambda calculus and computability, respectively, resolving Hilbert's Entscheidungsproblem negatively.[23][30] A classic example of an unsatisfiable formula is P∧¬PP \land \neg P, which yields a contradiction under any interpretation, as no assignment can make both PP and ¬P\neg P true simultaneously. Conversely, the implication P→QP \to Q (equivalent to ¬P∨Q\neg P \lor Q) is satisfiable—for instance, true when PP is false or QQ is true—but not valid, as it fails when PP is true and QQ is false. These illustrate how satisfiability captures existential model existence, distinct from the universal quantification in validity.[23]

References

  1. Validity and Invalidity, Soundness and Unsoundness
  2. [PDF] Validity and Soundness - rintintin.colorado.edu
  3. [PDF] Aristotle's Logic
  4. [PDF] Aristotle on the Syllogism and Its Parts - Thomas Aquinas College
  5. [PDF] Lecture Notes on Propositional Logic and Proofs
  6. [PDF] Propositional Logic
  7. Chapter 8 - Stanford Introduction to Logic
  8. [PDF] Satisfiability and Validity
  9. Classical Logic - Stanford Encyclopedia of Philosophy
  10. Logical Consequence - Stanford Encyclopedia of Philosophy
  11. Aristotle's Logic - Stanford Encyclopedia of Philosophy
  12. [PDF] Proving Invalidity Using Counterexamples - rintintin.colorado.edu
  13. [PDF] An argument with all true premises and a true conclusion, might or ...
  14. [PDF] Field Logical Validity - NYU Arts & Science
  15. Logic and the Study of Arguments – Critical Thinking
  16. Introduction to Logic for ProtoThinker - The Mind Project
  17. Deductive and Inductive Arguments - Philosophy Home Page
  18. [PDF] DEVELOPMENT OF LOGIC
  19. Deductive Systems in Traditional and Modern Logic - MDPI
  20. Validity and Soundness | Internet Encyclopedia of Philosophy
  21. [PDF] 1.2 — Forms and Validity - PHIL 240 Homepage
  22. [PDF] Argument Forms: Proving Invalidity - rintintin.colorado.edu
  23. Chapter Ten: How to Think About Deductive Logic
  24. [PDF] Introduction to Deductive Logic Part II: Argument Evaluation
  25. formal logic (informally)
  26. Propositional Logic - Stanford Encyclopedia of Philosophy
  27. Tarski on Logical Consequence - Project Euclid
  28. [PDF] Gödel's Completeness Theorem - Selmer Bringsjord
  29. [PDF] Semantical Analysis of Intuitionistic Logic I - Princeton University
  30. Introduction to Logic Truth, Validity, and Soundness
  31. Critical Thinking - Phil 106 - SIUE
  32. Logic, Epistemology and Argument Appraisal - SpringerLink
  33. The Rise and Fall of the Entscheidungsproblem
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