Nina Tod
§1. When a logician is asked whether an argument is strong, she typically checks its validity. In the Bayesian debate, however, classical logic is often considered inadequate as a measure of argument strength. There are invalid arguments that are strong and valid ones that are not, including informal fallacies such as circular arguments: ‘Lia is smart, so Lia is smart.’
In response, Bayesian measures aim to provide a more general normative standard for argument strength (Hahn 2020). The most prominent are confirmation-based: learning the premises of a strong argument raises the degree of belief in the conclusion. Hartmann and Trpin (2023) instead propose a coherence-based measure: in a strong argument, the premises cohere with the conclusion better than the negated premises do with the same conclusion.
§2. I raise challenges to these two measures. First, I present a counterexample to the coherence-based account:
A1: Pia is bored. ¬A1: Pia is not bored.
A2: Pia is not sad. ¬A2: Pia is not not sad.
C: Pia has a feeling C: Pia has a feeling.
The argument and its counterpart are clearly strong, yet – as I show in detail – the measure classifies one as weak or both as non-arguments.
Second, confirmation-based measures define strength via an agent’s belief change. But strength appears to be a property of the argument itself. A measure of persuasiveness should therefore explain which features make the argument persuasive. Logic explains this via structure and the meaning of logical constants, accounting for truth preservation.
§3. I defend a pluralist view: argument strength is multidimensional. ‘Lia is smart, so Lia is smart’ is strong with respect to necessary truth preservation but weak with respect to persuasiveness. Classical logic thus is not flawed: it captures one dimension, and does so successfully.
§4. In conclusion, while current Bayesian measures face serious challenges, the framework remains promising – not as a general account of argument strength, but as a measure of persuasiveness. Bayesian measures can coexist peacefully with classical logic.
Chair: tba
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SOPhiA 2026 