Peirce's law: ((A → B) → A) → A is a tautology
Peirce's law, ((A -> B) -> A) -> A, is a tautology with no visible negation. The check holds on all four rows, the simplifier finds no law to apply, and the normal-form converter reaches TRUE through implication elimination.
The formula says: if assuming that A implies B is enough to establish A, then A holds outright. It is famous because it is true on every row of the classical table yet uses only the arrow, and because it fails in logics that do not accept excluded middle. The engine's reading unpacks the nested arrows as an “if … then” whose condition is itself an “if … then”. On the four-row table over A and B the column is true everywhere, Σm(0, 1, 2, 3), with an empty maxterm list.
The tautology check holds: peirce is true on every assignment. The contradiction check is violated and satisfiability holds, with the witness A = FALSE, B = FALSE. The simplifier found no law that shortens the three-operator expression: no local rewrite matches the nested implications, so it reports the expression unchanged although the function is constant. The converter does reach the constant: with implication elimination three times, absorption and complement it returns TRUE as the CNF, and with an extra distribution step TRUE as the DNF. The Karnaugh map covers the four cells in one group and derives TRUE. Gates: 6 NAND, 9 NOR.
Peirce's law is the standard example that a formula can be a tautology without looking like one, which is the practical point for anyone reviewing a rule: the shape of an expression is not evidence about its truth table. In testing terms, a condition built only from implications can still be dead. The thing to watch is the gap between the simplifier's result and the check's verdict: the simplifier says nothing applies, the tautology check says always true, and both are correct about what they measure.
peirce := ((A -> B) -> A) -> A · output
Exact checks
Truth table (4 rows)
| # | A | B | peirce |
|---|---|---|---|
| 0 | FALSE | FALSE | TRUE |
| 1 | FALSE | TRUE | TRUE |
| 2 | TRUE | FALSE | TRUE |
| 3 | TRUE | TRUE | TRUE |
Plain-language reading
Beginnerpeirce is true exactly when the following “if … then” condition holds:
1. if: the following “if … then” condition holds:
1.1. if: if A is true, then B is true
1.2. then: A is true
2. then: A is true
In every other case, peirce is false.
About the operators:
- An “if … then” condition only constrains the case where its “if” part holds: when the “if” part is false, the condition is satisfied whatever the “then” part is. It does not say the reverse.
Intermediatepeirce is true exactly when the following “if … then” condition holds:
1. if: the following “if … then” condition holds:
1.1. if: if A is true, then B is true
1.2. then: A is true
2. then: A is true
In every other case, peirce is false.
Advancedpeirce is true exactly when the following “if … then” condition holds [material implication →]:
1. if: the following “if … then” condition holds [material implication →]:
1.1. if: if A is true, then B is true [material implication →]
1.2. then: A is true
2. then: A is true
In every other case, peirce is false.
Formal notation: ((A → B) → A) → A
Logic diagram
Simplification, step by step
The engine found no law that shortens this expression; it is already as short as its rewriting laws can make it.
((A -> B) -> A) -> A
Karnaugh map
- cells m0, m1, m2, m3
TRUE
Minterms and maxterms
peirce(A, B) = Σm(0, 1, 2, 3)peirce(A, B) = ΠM()Variable order: A, B · 4 rows
Normal forms
Conjunctive normal form (CNF)
TRUE0 clauses, 0 literals · complete · Implication elimination ×3, Absorption ×1, Complement ×1
Disjunctive normal form (DNF)
TRUE1 terms, 0 literals · complete · Implication elimination ×3, Distribution ×1, Absorption ×1, Complement ×1
NAND-only and NOR-only
NAND-only form
NAND(NAND(NAND(A, NAND(B, B)), NAND(A, A)), NAND(A, A))6 gates · verified equivalent on every row
NOR-only form
NOR(NOR(NOR(NOR(NOR(A, A), B), A), A), NOR(NOR(NOR(NOR(A, A), B), A), A))9 gates · verified equivalent on every row
The same rule as code
Python
# -*- coding: utf-8 -*-
# Generated by Tangibool (tangibool.python/1). Do not edit by hand; regenerate from the source revision.
# Model: not stated
# Source revision: reference
# Interpretation status: not stated
"""Tangibool evaluator (decision model).
Generated deterministically from the validated Boolean model; it evaluates exactly that
model. It does not establish that the model is a complete, legal, or safe statement of a
real-world rule, and evaluating examples does not verify the rule.
Source revision: reference
Interpretation status: not stated
Exporter: 1.0.0 (format tangibool.python/1; engine 1.0.0; semantics tangibool-semantics/1)
Logic digest: 37d6ea77bdbd2118a3b438a4b357a1d0c96db9631c1a2949506214cd859deaa7
Assumptions digest: d58ee1ad38e23e5093e4baa1047b5ae70d6972e08f555e34ac3fa16df63c4a78
Inputs (dict keys, declared order) and generated parameter names:
'A' -> input_A
'B' -> input_B
Outputs (declared order) and their functions:
'peirce' -> output_peirce(inputs)
Assumptions (the declared scope, checked by in_scope(inputs)):
none; every complete assignment is in scope.
Input contract:
Every required input must be exactly True or False (type(value) is bool). Missing
inputs, None, strings, integers (including 0 and 1), and keys that are not declared
inputs raise InputError before anything is evaluated, including inputs that a
short-circuiting operator would never read. Results are always bool.
Scope:
Decision functions raise OutOfScopeError when the inputs violate an assumption; an
out-of-scope assignment is never turned into a False result. Pass check_scope=False
to evaluate without the scope check, and use in_scope(inputs) to test it separately.
Usage:
import tangibool_model as model
model.evaluate({'A': False, 'B': False})
# -> {'peirce': True}
model.output_peirce({'A': False, 'B': False})
# -> True
model.evaluate({'A': 1, 'B': False})
# raises InputError: 1 is an int, not a bool
"""
import collections.abc as _abc
from typing import Dict, Mapping, Tuple
__all__ = [
"MODEL_INFO",
"IDENTIFIER_MAP",
"INPUT_NAMES",
"OUTPUT_NAMES",
"ASSUMPTIONS",
"InputError",
"OutOfScopeError",
"minterm_index",
"in_scope",
"evaluate",
"output_peirce",
]
MODEL_INFO = {
"format": "tangibool.python/1",
"mode": "decision",
"model": None,
"revision": "reference",
"interpretation": None,
"exporter": "1.0.0",
"engine_version": "1.0.0",
"semantics_version": "tangibool-semantics/1",
"logic_digest": "37d6ea77bdbd2118a3b438a4b357a1d0c96db9631c1a2949506214cd859deaa7",
"assumptions_digest": "d58ee1ad38e23e5093e4baa1047b5ae70d6972e08f555e34ac3fa16df63c4a78",
"primary_output": "peirce",
"unused_inputs": (),
"subcircuits": (),
}
# Stable ID -> generated identifier (reversible mapping; names are exact, escaped strings).
IDENTIFIER_MAP = {
"v_17179fa6144d": {"kind": "input", "name": "A", "identifier": "input_A"},
"v_69f16327c23f": {"kind": "input", "name": "B", "identifier": "input_B"},
"f_5842b6b189a1": {"kind": "output", "name": "peirce", "identifier": "output_peirce"},
}
INPUT_NAMES = ("A", "B")
OUTPUT_NAMES = ("peirce",)
ASSUMPTIONS = ()
_DECLARED_INPUTS = frozenset(INPUT_NAMES)
_SCOPE_INPUTS = ()
_ASSUMPTION_FORMULAS = {entry["name"]: entry["formula"] for entry in ASSUMPTIONS}
class InputError(ValueError):
"""Raised when inputs are missing, undeclared, or not exactly True/False.
Every required input must satisfy type(value) is bool. Missing inputs, None, strings,
integers (including 0 and 1), and keys that are not declared inputs are rejected before
anything is evaluated. A missing input is never treated as False. The individual problems
are listed in the problems attribute.
"""
def __init__(self, problems):
self.problems = tuple(problems)
super().__init__("Invalid inputs: " + "; ".join(self.problems))
class OutOfScopeError(ValueError):
"""Raised when the inputs violate the model's assumptions (its declared scope).
Outside its assumptions the rule has no decision: this error is never a False result.
The names of the violated assumptions are in the violated attribute.
"""
def __init__(self, violated):
self.violated = tuple(violated)
details = ", ".join(name + " (" + _ASSUMPTION_FORMULAS.get(name, "?") + ")" for name in self.violated)
super().__init__(
"Out of scope: the inputs violate "
+ ("assumption " if len(self.violated) == 1 else "assumptions ")
+ details
+ ". The rule has no decision outside its assumptions; this is not a False result."
)
def _short_repr(value: object) -> str:
text = repr(value)
return text if len(text) <= 60 else text[:57] + "..."
def _checked_inputs(inputs: Mapping[str, bool], required: Tuple[str, ...], check_scope: bool) -> Dict[str, bool]:
"""Validate inputs strictly before anything is evaluated and return a plain copy."""
if type(check_scope) is not bool:
raise TypeError("check_scope must be True or False")
if not isinstance(inputs, _abc.Mapping):
raise InputError(["inputs must be a mapping from input names to True/False, got " + type(inputs).__name__])
problems = []
for key in inputs:
if type(key) is not str or key not in _DECLARED_INPUTS:
declared = ", ".join(repr(name) for name in INPUT_NAMES) or "none"
problems.append("undeclared input " + _short_repr(key) + " (declared inputs: " + declared + ")")
needed = tuple(required) + (_SCOPE_INPUTS if check_scope else ())
for name in needed:
if name not in inputs:
problems.append("missing required input " + repr(name))
for key, value in inputs.items():
if type(key) is str and key in _DECLARED_INPUTS and type(value) is not bool:
problems.append(
"input " + repr(key) + " must be True or False (type bool), got "
+ type(value).__name__ + " " + _short_repr(value)
)
if problems:
raise InputError(list(dict.fromkeys(problems)))
return dict(inputs.items())
def _require_bool_args(function: str, pairs: Tuple[Tuple[str, object], ...]) -> None:
problems = [
"argument " + repr(name) + " of " + function + " must be True or False (type bool), got "
+ type(value).__name__ + " " + _short_repr(value)
for name, value in pairs
if type(value) is not bool
]
if problems:
raise InputError(problems)
def minterm_index(*values: bool) -> int:
"""Row index of an assignment over a declared variable order.
The first value is the most significant bit, so all-False is row 0. Used for indexed
minterm (sum) and maxterm (product) forms.
"""
index = 0
for value in values:
if type(value) is not bool:
raise InputError(["minterm_index() takes True/False values, got " + type(value).__name__ + " " + _short_repr(value)])
index = index * 2 + (1 if value else 0)
return index
def _scope_values(values: Dict[str, bool]) -> Tuple[Tuple[str, bool], ...]:
"""Value of every enabled, confirmed assumption for validated inputs, in declared order."""
return ()
def _require_in_scope(values: Dict[str, bool]) -> None:
violated = tuple(name for name, holds in _scope_values(values) if not holds)
if violated:
raise OutOfScopeError(violated)
def in_scope(inputs: Mapping[str, bool]) -> bool:
"""True when the inputs satisfy every assumption (the model's declared scope).
Requires the inputs the assumptions read; raises InputError for invalid inputs.
"""
values = _checked_inputs(inputs, (), True)
return all(holds for _, holds in _scope_values(values))
# ---- Formula bodies (inputs are validated by the public functions below) ----
def _compute_output_peirce(input_A: bool, input_B: bool) -> bool:
"""Decision output 'peirce': ((A -> B) -> A) -> A"""
return (not ((not ((not input_A) or input_B)) or input_A)) or input_A
# ---- Public API ----
def output_peirce(inputs: Mapping[str, bool], *, check_scope: bool = True) -> bool:
"""Decision output 'peirce' (stable ID f_5842b6b189a1).
'peirce' := ((A -> B) -> A) -> A
Required inputs: 'A', 'B'.
Raises InputError for missing, undeclared, or non-bool inputs, and OutOfScopeError
when check_scope is True and the inputs violate an assumption.
"""
values = _checked_inputs(inputs, ("A", "B"), check_scope)
if check_scope:
_require_in_scope(values)
output_peirce_value = _compute_output_peirce(values["A"], values["B"])
return output_peirce_value
def evaluate(inputs: Mapping[str, bool], *, check_scope: bool = True) -> Dict[str, bool]:
"""Evaluate every output (declared order); each output is computed once.
Required inputs: 'A', 'B'.
Raises InputError for missing, undeclared, or non-bool inputs, and OutOfScopeError
when check_scope is True and the inputs violate an assumption.
"""
values = _checked_inputs(inputs, ("A", "B"), check_scope)
if check_scope:
_require_in_scope(values)
output_peirce_value = _compute_output_peirce(values["A"], values["B"])
return {
"peirce": output_peirce_value,
}
PostgreSQL
-- Tangibool PostgreSQL export (tangibool.postgresql/1)
-- Target: PostgreSQL 16 or later with typed boolean inputs; tested with PostgreSQL 18.3 (PGlite 0.5.8).
-- Not a claim of compatibility with other SQL dialects.
-- Model: not stated
-- Source revision: reference
-- Interpretation status: not stated
-- Exporter: 1.0.0 (engine 1.0.0; semantics tangibool-semantics/1)
-- Logic digest: 37d6ea77bdbd2118a3b438a4b357a1d0c96db9631c1a2949506214cd859deaa7
-- Assumptions digest: d58ee1ad38e23e5093e4baa1047b5ae70d6972e08f555e34ac3fa16df63c4a78
-- Mode: decision
--
-- Read-only: this file contains only WITH / VALUES / SELECT statements. No DDL, DML,
-- procedures, credentials, network access, or database changes. The expressions are pure
-- Boolean expressions, so their result does not depend on SQL evaluation order.
--
-- NON-NULL INPUT PRECONDITION: NULL is not a truth value of this rule. Every fragment below
-- is defined only when every input column it reads is a non-NULL boolean. The guard template
-- (section 2) marks any NULL input as 'invalid_input' and any row that violates the
-- assumptions as 'out_of_scope', and fills output columns only for 'ok' rows; a NULL in an
-- output column of another row is an envelope marker, not a third truth value. Never wrap
-- the fragments in COALESCE(..., FALSE) or otherwise choose a meaning for unknown inputs.
-- This evaluates the supplied model; it does not show that the model is a complete, legal,
-- or safe statement of a real-world rule.
--
-- Input columns (stable ID: name -> column):
-- v_17179fa6144d: A -> "in_A"
-- v_69f16327c23f: B -> "in_B"
-- Output columns:
-- f_5842b6b189a1: peirce -> "out_peirce"
-- Assumptions (scope K; exported separately and never folded into an output):
-- none; every complete input row is in scope.
-- ----------------------------------------------------------------------------------------------
-- 1. Expression fragments (parenthesised; non-NULL boolean inputs required)
-- ----------------------------------------------------------------------------------------------
--
-- [out_peirce] output peirce (stable ID f_5842b6b189a1)
-- peirce := ((A -> B) -> A) -> A
-- PRECONDITION: "in_A", "in_B" are non-NULL booleans.
-- ((NOT ((NOT ((NOT "in_A") OR "in_B")) OR "in_A")) OR "in_A")
--
-- ----------------------------------------------------------------------------------------------
-- 2. Guard template (read-only SELECT over an inputs list)
-- ----------------------------------------------------------------------------------------------
-- Replace the example rows between the markers with your own rows, or replace the inputs
-- CTE with a SELECT that provides the boolean columns listed in the header. status is
-- 'invalid_input' when any input is NULL, 'out_of_scope' when the assumptions are false, and
-- 'ok' otherwise; output columns are filled only for 'ok' rows (NULL elsewhere is an
-- envelope marker). Assumption columns are shown for rows with complete inputs.
WITH inputs ("row_id", "in_A", "in_B") AS (
-- tangibool:example-rows:begin
VALUES
(1, FALSE, FALSE),
(2, FALSE, TRUE),
(3, TRUE, FALSE),
(4, TRUE, TRUE),
(5, NULL::boolean, FALSE)
-- tangibool:example-rows:end
),
checked AS (
SELECT
inputs.*,
CASE
WHEN "in_A" IS NULL OR "in_B" IS NULL THEN 'invalid_input'
ELSE 'ok'
END AS "status"
FROM inputs
)
SELECT
"row_id",
"in_A",
"in_B",
"status",
CASE WHEN "status" = 'ok' THEN ((NOT ((NOT ((NOT "in_A") OR "in_B")) OR "in_A")) OR "in_A") END AS "out_peirce"
FROM checked
ORDER BY "row_id";
SMT-LIB
; Tangibool SMT-LIB export (tangibool.smtlib/1) ; Profile: SMT-LIB 2.6 Boolean subset (QF_UF): Bool constants, define-fun, let, push/pop, assert, check-sat, echo. ; Tested with Z3 5.1.0 (z3-solver 5.2.0, WebAssembly build). ; Model: not stated ; Source revision: reference ; Interpretation status: not stated ; Exporter: 1.0.0 (engine 1.0.0; semantics tangibool-semantics/1) ; Logic digest: 37d6ea77bdbd2118a3b438a4b357a1d0c96db9631c1a2949506214cd859deaa7 ; Assumptions digest: d58ee1ad38e23e5093e4baa1047b5ae70d6972e08f555e34ac3fa16df63c4a78 ; Mode: decision ; ; Reading results: each query block states what sat and unsat mean for its question. A sat ; answer comes with a witness or counterexample (ask for it with (get-model) after sat only); ; unsat settles that scoped query and is solver-checked, not an independently checked proof ; certificate; unknown settles nothing. Blocks are scoped with push/pop, so assertions never ; accumulate across questions. This checks the supplied model, not whether it is a complete, ; legal, or safe statement of a real-world rule. ; ; Symbols (stable ID: name -> symbol): ; v_17179fa6144d: A -> in_A ; v_69f16327c23f: B -> in_B ; f_5842b6b189a1: peirce -> out_peirce (set-option :produce-models true) (set-info :smt-lib-version 2.6) (set-logic QF_UF) ; ---- Inputs (declared order) (declare-const in_A Bool) ; A (declare-const in_B Bool) ; B ; ---- Outputs (dependencies first) ; output peirce (stable ID f_5842b6b189a1): ((A -> B) -> A) -> A (define-fun out_peirce () Bool (=> (=> (=> in_A in_B) in_A) in_A)) ; ---- Assumptions (the scope K; never folded into an output) ; K: conjunction of the enabled, confirmed assumptions (true when there are none) (define-fun K () Bool true) ; ---- Definitions only: nothing is asserted and no check is run. ; Add (assert ...) and (check-sat) inside (push 1) ... (pop 1) to ask a question.
LaTeX
peirce = \left(\left(A \rightarrow B\right) \rightarrow A\right) \rightarrow A
The common slip
Concluding from the simplifier's unchanged result that the formula is not a tautology. The simplifier only reports that none of its named laws applies; it does not evaluate the table. The tautology check does, and it holds on all four rows, as does the converter that reduced the formula to TRUE.
Questions
How does the converter get to TRUE?
It rewrites each arrow by implication elimination, three steps, pushes negations inward, and finds A OR NOT A inside the result, which the complement law turns into TRUE after an absorption. The step list on the page names each law, and the result is checked against the table.
Related
Operators and gates
Tools
- Truth table generator: any Boolean expression, up to 12 variables, with intermediate columns
- Boolean algebra calculator and simplifier with steps (every law named and verified)
- Karnaugh map solver: 2 to 4 variables, groups, SOP and POS, don’t-cares from assumptions
See also
- A tautology: A OR NOT A, checked on every row
- Tautology in logic: meaning and examples
- Implication elimination: A → B = NOT A OR B, and the →E rule
- Implication as NOT A OR B, verified
- Tautology, contradiction or contingency: exercises with answers
- “Simplified” is not “minimal”: what a simplifier can promise
Worked examples
Generated from the Tangibool engine, semantics tangibool-semantics/1, engine 1.0.0, build . Every table, map, simplification and verdict on these pages was computed by the Tangibool engine when the site was built, and every claim was re-checked by the same exact checker the app uses. Limits are stated where they apply. © 2026 Foreranger LTD.