praeiudicium confirmationis
Confirmation bias is the systematic privileging of evidence that supports an already held proposition, while evidence against it is discounted, ignored, or never sought. The belief arrives first and thereafter conducts its own trial.
Taxonomy - Evidential weighting bias, belief-preserving distortion. The error is not holding a prior belief; it is letting that belief govern the admissibility and weight of everything that follows.
Informal Example - "Every source that agrees with my thesis is reliable; every source that disagrees is biased."
Canonical Form:
B(P) ∧ E+(P) ∧ E-(P) ⊢ overweight(E+) ∧ underweight(E-)
Where:
No single deductive step fails; the distortion is in the weighing, and every weighing leans the same way.
Invalid Update Pattern:
Weight(E | P) = f(Agreement(E,P))
Weight is assigned by agreement with the conclusion, not by reliability, relevance, or independence. The evidence is graded on loyalty.
Missing Necessary Condition
Rational assessment requires symmetrical criteria for admissibility and defeat:
Criteria(E+) = Criteria(E-)
One standard, both directions; the moment the criteria fork, the inquiry has ended.
Biased Model:
B(P) → Select(E+) ∧ Suppress(E-)
Corrective Model:
Reliability(E) ∧ Relevance(E,P) → Weight(E)
Weight must come from evidential quality; agreement with an antecedent belief is not a quality of the evidence.
Rational updating requires:
P(P | E) = P(E | P)P(P) / P(E)
The biased pattern effectively changes the likelihood term by preference:
Pb(E | P) > P(E | P) when E confirms P
and decreases evidential force when E threatens P.
𝔼(E+) ∧ Ignore(E-) ⇏ □(P)
Selective evidence cannot justify certainty when the defeaters were excluded by preference; the conclusion was reached before the evidence arrived.
Confirming Evidence With Defeater
E+(P) ∧ E-(P) ∧ E- defeats E+
The conclusion may be unsupported once omitted evidence is restored.
Independent Search
Search(E) independent of B(P)
An unbiased inquiry finds the evidence first and sorts it afterward; the bias sorts first.
The modern label is recent, but the logical concern is older. Bacon's Novum Organum describes idols of the mind, especially idola specus and idola tribus, as sources of distorted inquiry.
A close historical parallel is the older concern that the intellect preserves what it has once accepted and bends new material toward it. This is not a canonical Latin name for confirmation bias, but it is a recognizable ancestor of the logical problem.
The Neo-Latin phrase praeiudicium confirmationis is descriptive rather than traditional: useful as a label, but not a claim of classical taxonomy.
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