First-order logic with quantifiers, predicates, functions, and identity. Covers syntax, interpretation over a domain, the universal and existential quantifiers, quantifier scope and binding, translation from natural language with multi-variable predicates, natural deduction for quantifiers, undecidability at first order, and the soundness/completeness theorems. Use when propositional logic is insufficient -- whenever "for all" or "there exists" matters to the argument.
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First-order logic with quantifiers, predicates, functions, and identity. Covers syntax, interpretation over a domain, the universal and existential quantifiers, quantifier scope and binding, translation from natural language with multi-variable predicates, natural deduction for quantifiers, undecidability at first order, and the soundness/completeness theorems. Use when propositional logic is insufficient -- whenever "for all" or "there exists" matters to the argument.
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skill
category
logic
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stable
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"2026-04-12T00:00:00.000Z"
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examples/skills/logic/predicate-logic/SKILL.md
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Predicate Logic (First-Order Logic)
Predicate logic -- also called first-order logic (FOL) or quantificational logic -- extends propositional logic with the machinery of quantification. Where propositional logic can express "Socrates is mortal" as an atomic sentence, predicate logic can express "every man is mortal" as ∀x (Man(x) → Mortal(x)) and then actually derive "Socrates is mortal" from it. This expressive jump is what allows logic to capture mathematical reasoning, natural-language quantifier interactions, and the semantics of programming languages. This skill covers the syntax, semantics, proof theory, and translation discipline of first-order logic.
Agent affinity: frege (foundational framing), russell (type-theoretic discipline), tarski (semantic definition of truth)
Terms are built from variables, constants, and function applications. If f is a 2-place function symbol, then f(x, a) is a term.
Atomic formulas are predicate applications: P(t) or R(t, s), where t, s are terms.
Formulas are built from atomic formulas using the propositional connectives plus the two quantifier rules:
If φ is a formula and x is a variable, then ∀x φ is a formula
If φ is a formula and x is a variable, then ∃x φ is a formula
Free and bound variables. A variable x in a formula is bound if it lies inside the scope of a quantifier ∀x or ∃x. Otherwise it is free. A formula with no free variables is a sentence -- it has a definite truth value once the interpretation is fixed. A formula with free variables is open -- it is a property that some objects may satisfy.
Semantics: Interpretation Over a Domain
An interpretation of a first-order language consists of:
A nonempty domain (also called universe) D of objects
For each constant c, an object c^I in D
For each n-place function symbol f, a function f^I : D^n → D
For each n-place predicate symbol P, a relation P^I ⊆ D^n
Given an interpretation and an assignment of domain elements to the free variables, every formula has a truth value, determined recursively:
∀x φ is true if φ is true for every assignment of x to an element of D
∃x φ is true if φ is true for at least one assignment of x to an element of D
This is Tarski's inductive definition of truth, which is why Tarski is the semantics specialist in this department.
The Two Quantifiers
Universal quantifier ∀
"For every x in the domain, property φ(x) holds."
In natural language: "all," "every," "any," "each," and sometimes "a" or "the" (depending on context -- "a whale is a mammal" usually means "every whale is a mammal").
Translation pattern: ∀x (P(x) → Q(x)). Not ∀x (P(x) ∧ Q(x)) -- that says "every object in the domain has both P and Q," which is almost never what English "every P is Q" means.
Existential quantifier ∃
"There is at least one x in the domain such that property φ(x) holds."
In natural language: "some," "there is," "there exists," "at least one," and sometimes "a" or "the" (again context-dependent -- "a student failed" usually means "some student failed").
Translation pattern: ∃x (P(x) ∧ Q(x)). Not ∃x (P(x) → Q(x)) -- that is vacuously true as long as any object lacks P.
The translation asymmetry (universal uses →, existential uses ∧) is a persistent source of student confusion and is worth drilling until reflexive.
Quantifier Scope and Alternation
Scope
A quantifier's scope is the formula it governs, usually the smallest formula immediately following. Parentheses disambiguate. In ∀x (P(x) → Q(x)), the scope of ∀x is the full conditional. In ∀x P(x) → Q(x), the scope is only P(x), and Q(x) has a free variable.
Alternation order matters
∀x ∃y Loves(x, y) means: for every person, there is someone they love. Different person y for different x.
∃y ∀x Loves(x, y) means: there is one person y that everyone loves. Same y for every x.
These are not equivalent. In fact, ∃y ∀x φ(x, y)implies∀x ∃y φ(x, y) but the reverse does not hold. This is the single deepest pitfall of quantifier logic.
Quantifier duality (De Morgan for quantifiers)
¬∀x φ
≡ ∃x ¬φ
¬∃x φ
≡ ∀x ¬φ
¬∀x P(x)
"Not everything is P" ≡ "something is not P"
¬∃x P(x)
"Nothing is P" ≡ "everything is not P"
Worked Example: Aristotle's Syllogism
Argument: All humans are mortal. Socrates is human. Therefore, Socrates is mortal.
Formalization: Let Human(x), Mortal(x) be one-place predicates, and let s denote Socrates.
Human(s) → Mortal(s) (Universal elimination from 1, substituting s for x)
Mortal(s) (Modus ponens from 3 and 2)
This is the paradigm derivation of predicate logic. It looks trivial only because we have seen it a thousand times.
Natural Deduction for Quantifiers
Universal introduction (∀I)
If you can prove φ(x) for an arbitrary x (meaning x does not appear in any undischarged assumptions), you can conclude ∀x φ(x).
The "arbitrary" restriction is critical. You cannot introduce ∀ if you have been using specific properties of x.
Universal elimination (∀E)
From ∀x φ(x), conclude φ(t) for any term t.
Substitution must be capture-avoiding: if t contains a variable y, and y would become bound by a quantifier in φ, rename bound variables first.
Existential introduction (∃I)
From φ(t) for a specific term t, conclude ∃x φ(x).
Existential elimination (∃E)
From ∃x φ(x), introduce a fresh variable (or constant) c and assume φ(c). If you can derive ψ from this assumption without using c outside the sub-derivation, conclude ψ.
The "fresh" restriction is critical. You cannot use ∃E on a variable that appears elsewhere.
Translation Discipline
Translating English to FOL requires careful attention:
"Every student passed" uses the restricted quantifier over students. Formalize: ∀x (Student(x) → Passed(x)).
Two-place predicates
"Alice loves Bob" → Loves(a, b) with constants a, b.
"Everyone loves Bob" → ∀x Loves(x, b).
"Alice loves everyone" → ∀x Loves(a, x).
"Everyone loves someone" → ∀x ∃y Loves(x, y) (different someone for different x).
"Someone loves everyone" → ∃y ∀x Loves(x, y) (same y for every x).
Uniqueness and numerical quantifiers
"There is exactly one x such that P(x)" → ∃x (P(x) ∧ ∀y (P(y) → y = x)).
"There are at least two P" → ∃x ∃y (P(x) ∧ P(y) ∧ x ≠ y).
"There are exactly two P" → combine both: at least two and at most two.
Soundness and Completeness
Soundness: If a formula is provable in the deduction system, it is true in every interpretation. This guarantees you cannot derive a false conclusion from true premises.
Completeness: If a formula is true in every interpretation, it is provable. This is Godel's completeness theorem for first-order logic (1929, dissertation; published 1930) -- not to be confused with the incompleteness theorems, which are about specific theories within first-order logic.
The soundness + completeness pair means that "true in every interpretation" and "derivable in the deduction system" are exactly the same set. This is philosophically significant: it means first-order logic captures its own semantic notion of validity.
Undecidability at First Order
Propositional logic is decidable. First-order logic is undecidable -- there is no algorithm that, given any first-order formula, always correctly determines whether it is valid in finite time. This is Church's theorem (1936) and independently Turing's proof via the halting problem.
First-order logic is semi-decidable: if a formula is valid, you can find a proof (given enough time), but if it is not valid, the search may run forever without terminating.
This is the first-order inflection point where logic meets computability theory. It is not a minor technical detail -- it sets the limits of what automated reasoning can do.
When NOT to Use This Skill
Arguments with no quantifier structure. Use propositional-logic.
Arguments essentially involving necessity or possibility. Use modal-logic.
Rhetorical error classification. Use informal-fallacies.
Mathematical proofs that use FOL as a tool but are really about math. Use mathematical-proof-logic.
Decision Guidance
When formalizing an argument into FOL:
Identify the domain. What are the objects? Humans? Numbers? Sets?
Identify the predicates. What properties and relations are being asserted?
Identify the quantifier structure. Where is "every," "some," "none"? What is the scope of each quantifier?
Translate carefully. All-is-→, some-is-∧, no requires negation. Alternation order matters.
Derive or countermodel. For valid arguments, give a natural-deduction proof. For invalid arguments, exhibit an interpretation where premises are true and conclusion false.
Cross-References
frege agent: Invention of quantifiers, Begriffsschrift, foundational framing
russell agent: Type discipline, Russell's paradox, Principia
tarski agent: Semantic definition of truth, model theory
propositional-logic skill: The base system FOL extends
mathematical-proof-logic skill: FOL as the working language of proof
References
Enderton, H. B. (2001). A Mathematical Introduction to Logic. 2nd edition. Academic Press.
Mendelson, E. (2015). Introduction to Mathematical Logic. 6th edition. CRC Press.
Frege, G. (1879). Begriffsschrift. Halle.
Tarski, A. (1933/1956). "The Concept of Truth in Formalized Languages," in Logic, Semantics, Metamathematics. Oxford University Press.
Church, A. (1936). "An unsolvable problem of elementary number theory." American Journal of Mathematics 58, 345-363.
Godel, K. (1930). "Die Vollstandigkeit der Axiome des logischen Funktionenkalkuls." Monatshefte fur Mathematik und Physik 37, 349-360.
