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GuidePublished 12 Aug 2026Updated 13 Aug 20268 min readBy Kevin Jogin
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Connections with Model Theory

Satisfaction and the Tarski Truth Definition

The recursive definition of truth in a structure, and the reason a recursion over formulas requires assignments rather than sentences alone.

Category Engineering / MathematicsSource V.1Pages 221-226Reading 2 minReviewed 2026-08-07

Learning objectives

  • State the satisfaction relation by recursion
  • Explain why assignments are needed
  • Compute satisfaction in concrete examples
On this page
  1. Assignments
  2. The recursion
  3. Worked satisfaction
  4. Two turnstiles

Assignments

Definition — Assignment

A function from the variables into the universe of a structure. Given an assignment, every term denotes an element and every formula receives a truth value.

Satisfaction is written A ⊧ Φ[a] — the structure A satisfies Φ under the assignment a.

Why assignments are unavoidable

A sentence has no free variables, so its truth is assignment-independent. But the recursion defining truth passes through subformulas that do have free variables — ∀x Φ is defined in terms of Φ, which has x free. So the recursion must be carried out for formulas with assignments even if only sentences are ultimately of interest. This is Tarski's insight.

The recursion

The satisfaction clauses
Formula<strong>A</strong> &#8871; &Phi;[<em>a</em>] holds when
t1 ≈ t2t1 and t2 denote the same element under a
r(t1,…)the denoted tuple lies in rA
¬ΦA ⊧ Φ[a] fails
Φ ∧ Ψboth hold
Φ ∨ Ψat least one holds
Φ → ΨΦ fails or Ψ holds
∀x ΦΦ holds under every assignment differing from a at most at x
∃x ΦΦ holds under some such assignment
The definition is materially adequate

Tarski's criterion is that the definition should yield, for each sentence, exactly the expected biconditional — that “snow is white” is true if and only if snow is white. The recursion above satisfies this, and it is what makes model theory possible as mathematics rather than philosophy.

Worked satisfaction

In the integers under addition

  • ∀x∃y (x + y ≈ 0) — true; every integer has an additive inverse.
  • ∃x∀y (x + y ≈ y) — true, witnessed by 0.
  • ∀x∃y (y + y ≈ x) — false; odd integers are not doubles.
Quantifier order matters

The second example is true; reversing the quantifiers to ∀y∃x gives a different and weaker statement that is also true, but the two are not equivalent in general. Reading quantifier prefixes carefully is the single most common source of error.

Two turnstiles

<strong>A</strong> &#8871; &Phi;
the structure satisfies the sentence
&Sigma; &#8871; &Phi;
every model of Σ satisfies Φ — semantic consequence
&Sigma; &#8866; &Phi;
Φ is derivable from Σ — syntactic consequence

Gödel's completeness theorem asserts that the last two coincide for first-order logic, exactly as Birkhoff's completeness theorem does for equational logic. The source does not prove Gödel's theorem but uses its consequences freely.

The equational case as a special case

For identities, A ⊧ p ≈ q in the Chapter II sense agrees with the Chapter V sense applied to the universally quantified sentence. The notation is reused deliberately.

Frequently asked questions

Why is the definition called a truth definition rather than a truth theory?

Because it defines truth for a fixed structure in a metalanguage, rather than analysing truth in general. Tarski showed a language cannot define its own truth predicate, which is why the metalanguage is essential.

Does satisfaction depend on the whole assignment?

Only on the values assigned to the free variables of the formula. This is a lemma proved by induction, and it is what makes sentences assignment-independent.

Related pages

  • First-Order Structures and Interpretation
  • Elementary Equivalence and Elementary Substructures
  • Identities, Satisfaction and Equational Classes

Source. S. Burris and H. P. Sankappanavar, A Course in Universal Algebra, The Millennium Edition — a corrected re-typesetting of Springer-Verlag Graduate Texts in Mathematics 78 (1981). Section V.1, book pages 221-226.

This page is an original exposition prepared for the KEVOS® knowledge library. It restates and reorganises mathematical results; it is not a reproduction of the source text.

Handbook application: from concept to controlled practice

Purpose. This expanded section turns the original page into a practical handbook. It preserves the supplied material and adds a repeatable way to apply, check and review Satisfaction and the Tarski Truth Definition. It does not replace a contract, legislation, a controlled standard, competent engineering judgement or specialist advice.

The operating aim is to turn a compact mathematical statement into a usable chain of definitions, claims, examples and checks. Read the original explanation first, then use the workflow and checks below to convert knowledge into evidence.

Treat Satisfaction and the Tarski Truth Definition as a network of definitions and implications, not as a list of formulas. The working vocabulary on this page—satisfaction, truth, definition, recursion, assignments—should be made explicit before any proof or computation begins. Record the ambient set or structure, the permitted operations and the equality or equivalence relation in use. A compact theorem often changes meaning when the base field, finiteness condition, commutativity assumption or direction of an action changes.

For a proof, write the hypotheses as a checklist and mark the line at which each one is used. For a computation, state the representation of the input, the arithmetic model, the termination condition and the output invariant. For a classification problem, distinguish existence from uniqueness and distinguish an object from its representation. These separations prevent a correct local calculation from being mistaken for the general result.

A useful worked example should be small enough to inspect completely but rich enough to exercise the main mechanism. Compute the result in two ways where practical: symbolically and by substitution, structurally and numerically, or directly and through a normal form. Then include one near-miss example in which a hypothesis fails. The contrast explains why the theorem is shaped as it is and gives the reader a diagnostic pattern for later problems.

Verification is part of the mathematics. Check domains and codomains, substitute proposed solutions, test identity and zero cases, compare dimensions or cardinalities, and confirm that maps respect the required operations. In numerical work, report precision, conditioning and a residual rather than digits alone. In algorithmic work, separate mathematical correctness from implementation complexity and resource limits.

Step-by-step operating method

  1. Fix the setting. State the objects, ambient structure, notation and assumptions before manipulating symbols.
  2. Separate claims. Distinguish definitions, hypotheses, conclusions, equivalent conditions and consequences.
  3. Choose a method. Select proof, construction, calculation or algorithm according to the question actually asked.
  4. Work a small case. Use the smallest non-trivial example to expose the mechanism and test edge behaviour.
  5. Verify independently. Substitute back, check invariants, test boundary cases or use an alternative derivation.

Worked-example protocol

Illustrative method—not a source theorem. Start with a small admissible input and list the definitions it must satisfy. Carry out each transformation on a separate line, citing the property that permits it. Preserve exact values until approximation is necessary. At the end, verify the output against the original definition and one invariant such as dimension, degree, determinant, order, norm or residual. Then alter one hypothesis and observe which step ceases to be valid. This protocol creates a reusable example without inventing a theorem-specific numerical answer.

StageRecordQuality check
InputObjects, domain, notation, assumptionsEvery symbol is defined
MethodPermitted operation or cited result at each stepAll hypotheses hold
OutputExact result and representationCorrect type, domain and form
VerificationSubstitution, invariant or alternative derivationIndependent agreement
Boundary testZero, identity, degenerate or failed hypothesisScope is understood

Common failure modes and recovery actions

1. Watch for

Using a theorem without checking every hypothesis.

Recovery: Return to the governing definition or requirement and restate the decision in one sentence.

2. Watch for

Treating a suggestive example as a proof of the general case.

Recovery: Separate evidence from assumption, assign an owner and set a date for validation.

3. Watch for

Changing notation or conventions part-way through an argument.

Recovery: Run a small counterexample, boundary test, pilot or independent check before proceeding.

4. Watch for

Hiding a division-by-zero, convergence, finiteness or commutativity assumption.

Recovery: Record the consequence, decision and rationale, then update the controlled baseline.

5. Watch for

Reporting a computed result without a residual, substitution or structural check.

Recovery: Escalate when the issue affects safety, compliance, acceptance, material value or an agreed tolerance.

Review checklist

  • Can every symbol be traced to a definition or prior result?
  • Which hypothesis does each major step use?
  • Does the method cover zero, identity, degenerate and boundary cases?
  • Can the conclusion be checked by a second representation or calculation?
  • Are mandatory requirements distinguished from recommendations and illustrative values?
  • Are sources, assumptions, units, dates and versions recorded closely enough to reproduce the decision?
  • Have safety, legal, ethical, stakeholder and operational consequences been considered at the appropriate level?
  • Is there a named owner and a trigger for review, escalation, change or retirement?

Questions for deeper application

What is the most important distinction a practitioner must preserve when applying Satisfaction and the Tarski Truth Definition?

Answer with a fact or cited source where available. Where evidence is incomplete, record the assumption, consequence, responsible owner and next validation action.

Which assumption about satisfaction would change the result most if it proved false?

Answer with a fact or cited source where available. Where evidence is incomplete, record the assumption, consequence, responsible owner and next validation action.

What evidence would allow an independent reviewer to reproduce or challenge the conclusion?

Answer with a fact or cited source where available. Where evidence is incomplete, record the assumption, consequence, responsible owner and next validation action.

Which boundary, exception or failure case has not yet been tested?

Answer with a fact or cited source where available. Where evidence is incomplete, record the assumption, consequence, responsible owner and next validation action.

What must be handed over, monitored or reviewed after the immediate work is complete?

Answer with a fact or cited source where available. Where evidence is incomplete, record the assumption, consequence, responsible owner and next validation action.

Authoritative references and use notes

The sources below were selected as institutional or primary guidance for the broader practice. They support the handbook method; they do not imply that every statement or clause in a source applies to every project. Confirm the current edition, jurisdiction, contract and application before treating any requirement as mandatory.

  • MIT OpenCourseWare — Algebra I — Massachusetts Institute of Technology. Used for groups, vector spaces, linear transformations and linear groups. Accessed 2026-08-13.
  • The Stacks Project — table of contents — The Stacks Project. Used for commutative algebra, homological algebra, modules and derived categories. Accessed 2026-08-13.

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