A Bayesian Account of Argumentation

Part of the Bayesian Argumentation series

Quantifying Argument

What makes for a good argument?

From a logical point of view, a good argument is logically sound. But in the real-world people rarely argue with pure logic.

From a rhetorical point of view, a good argument is one that is convincing. But how can this be measured?

In this series of essays, we present a Bayesian model of argumentation, where arguments are treated as information that may cause a Bayesian rational agent to modify their beliefs. Given a model of the beliefs of some Bayesian “subject”, we can objectively measure subjective aspects of an argument’s quality or strength, such as relevance, persuasiveness, and sufficiency.

This perspective can provide some powerful insights about argumentation for people working in artificial intelligence, law, argument mapping software, or in our case, design of social protocols.

Consider the following example.

Introductory Example 1

Consider the argument this is a good candidate for the job because he has a pulse.

If our subject is a Bayesian rational agent with common sense, then probably:

  • The argument is not very persuasive.
  • Nor is it informative. He has a pulse is probably not new information to the subject.
  • Yet the argument is clearly relevant, because:
    • If the subject learned that the subject did not have a pulse, this would be sufficient to reject him as a candidate.
    • Alternatively, the belief that he probably has a pulse is necessary for the belief that he might be a good candidate.

As everyone knows, a Bayesian rational agent updates their beliefs when they acquire new information. An argument that is not informative to the agent therefore can’t be persuasive. But a Bayesian model allows us to calculate what an agent would believe if they had different information (e.g. if they thought that the candidate didn’t have a pulse). This simple insight cracks open a number of ways of measuring argument strength other than just persuasiveness. In the essays on Relevance and Corelevance and Necessity and Sufficiency we will define these measures and see how they all relate mathematically to the informativeness of the argument.

The Bayesian model of argumentation also allows us to take into account the reliability of the arguer themselves. A Bayesian rational agent will only update their beliefs if they believe the information given them. So for an argument to be informative it must not just be new information, it must also be believable. This perspective shows us when what looks like an ad hominim fallacy may somteimes be perfectly rational1. These ideas are discussed in the essays on informativeness and persuasiveness.

Introductory Example 2

Now consider another example argument: the car won’t start because the car is out of gas. If the subject previously believed the car had gas, then this new information might well be persuasive. But suppose the subject accepts this information, but also believes that the car’s battery is dead? With this assumption, the car being out of gas is now in a sense irrelevant.

Clearly the relevance of an argument depends on context: it depends on other beliefs the subject has about the state of world.

Theoretically, if we have a model of some subject’s beliefs about the world, we can identify the corelevant beliefs – the beliefs cause the argument to be relevant. We will define this more precisely in the essay on relevance and corelevance.

Why Bayesian Argumentation?

Like all models, the Bayesian model of subjective belief is an incomplete description of the human mind. But it is clearly defined. Building clear terminology on top of a clear model helps clarify our thinking, facilitate discussion, and sharpen our intuition about what argument actually is.

There is a lot of recent academic work on Bayesian argumentation23. These essay are intended not as an overview of current theory, but as a useful set of definitions and formulas for practitioners: specifically software engineers building practical applications of argumentation for AI, argument mapping systems, or in our particular case, design of social protocols. Our goal is to provide a useful and clear vocabulary, with common-sense but precise definitions for common concepts related to argument strength. This can hopefully help clarify discussion among collaborators and prove useful in documentation and code.

Argumentation Theory and Warrants

This idea of unexpressed beliefs that justify an argument evokes the idea of the warrant from the field of argumentation theory. Argumentation theory views argument as a kind of flexible, informal logic. People don’t argue with logical syllogisms – instead they make simple statements, or claims which support other claims. For example, I might claim people are wearing jackets to support the claim it’s probably cold outside.

We use the terms premise and conclusion to differentiate between the supporting and supported claims. An argument is just a premise stated in support of some conclusion.

In every argument there is an unstated claim that this premise supports this conclusion. This doesn’t need to be stated because it’s implied by the fact that the argument was made. After asserting people are wearing jackets in support of the conclusion it’s probably cold outside, I don’t need to add, pedantically, “and you see, if people are wearing jackets it must be cold outside”.

This unexpressed premise that justifies the inferential leap from premise to conclusion is called the warrant.

The warrant doesn’t have to be a logical formula such as “if people are wearing jackets it must be cold outside”. It can be based on any kind of inferential rule (deductive, inductive, intuitive) or argumentation scheme (authority, analogy, example) – whatever justifies the inference in the mind of the arguer. Some academics use different terms for these concepts: our terminology is influenced by the influential Toulmin model, except we prefer the traditional terms premise and conclusion over grounds and claim. More precise definitions of our terms are given in the Deliberati Argument Model.

A Bayesian Definition of Warrant

In Bayesian terms, a rational agent is said to acquire evidence, which causes them to change their belief in the probability of some hypothesis (see this Bayesian Inference Primer).

There is clearly an analogy here: evidence is premise as to hypothesis is to conclusion. But argumentation theory also has warrants. What is the warrant in a Bayesian model?

The warrant clearly has to do with the subject’s prior beliefs, because a Bayesian agent’s priors are precisely what justify, in their mind, any inferential leap from premise to conclusion.

For example, if our subject is more likely to believe that (𝐴) it is going to rain today if they believe that (𝐵) the sky is cloudy than if they do not, then there clearly exists a warrant justifying, in the subject’s mind, the inferential leap from 𝐵 to 𝐴.

But why does this warrant exist in the subject’s mind? What is the inferential rule that actually justifies the inference? Is it a deductive inference? Inductive? Gut feeling?

We can’t necessarily answer this question, because a Bayesian agent’s beliefs are modeled by a simple probability distribution, which gives us the end result of the agent’s internal belief structure, but not how they got there.

If the prior beliefs of our subject are represented by the probability measure $P$, then we can at least say that, in the mind of the subject, a warrant exists justifying the inference from premise 𝐵 to conclusion 𝐴 iff:

$$ P(A|B) ≠ P(A|\bar{B}) $$

If the warrant exists, we say that 𝐵 is relevant to 𝐴. Otherwise, we say it is irrelevant.


So in a Bayesian argument, an arguer asserts a premise in support/opposition to some conclusion, and if the premise is relevant – the subject is more likely to believe the conclusion if they believe the premise – then there must be some warrant justifying the inference from premise to conclusion.

This Series

In the next essay in this series, we will formally define a measure of relevance from a Bayesian perspective and discuss some of its mathematical properties. In the remaining articles in this series we will define measures of necessity, sufficiency, informativeness, and persuasiveness, all of which relate back to this central concept of relevance.

Summary of Definitions

Below is a summary of all the terms that will be defined in the above essays.

For an argument with premise 𝐵 and conclusion 𝐴, and a subject whose beliefs are represented by probability measure P…

  • Relevant: The premise is relevant to the conclusion (or, the argument is relevant) iff $P(A \vert B) ≠ P(A \vert \bar{B})$

    • Otherwise, the premise is irrelevant to the conclusion (or, the argument is irrelevant)
      • Irrelevance implies statistical independence of A and B.
  • Support: The premise supports the conclusion iff $P(A \vert B) > P(A \vert \bar{B})$

  • Oppose: The premise opposes the conclusion iff $P(A \vert B) < P(A \vert \bar{B})$

    • If 𝐵 supports 𝐴, then 𝐵 opposes $\bar{A}$
  • Relevance: The relevance of the premise to the conclusion is $R(A,B) = P(A \vert B) - P(A \vert \bar{B})$

  • Conditional Relevance: Given some third premise 𝐶: $R(A,B \vert C) = P(A \vert B,C) - P(A \vert \bar{B},C)$

  • Corelevant: The premises 𝐵 and 𝐶 are corelevant to the conclusion 𝐴 iff: $R(A,B \vert C) ≠ R(A,B \vert \bar{C})$

  • Corelevance: $CR(A;B,C) = R(A,B \vert C) - R(A,B \vert \bar{C}) = R(A,C \vert B) - R(A,C \vert \bar{B})$

  • Necessity: The necessity of the premise to the conclusion is $N(A,B) = P(A) - P(A \vert \bar{B}) = P(B)R(A,B)$

  • Sufficiency: The sufficiency of the premise for the conclusion is $S(A,B) = P(A \vert B) - P(A) = P(\bar{B})R(A,B)$

  • Testimony Event: The event, directly observed by the subject, that the arguer asserted the premise in support of the conclusion.

  • Post-Argument Belief: Given the testimony event I: $P_i(∙) = P(∙ \vert I)$

    • e.g. $P_i(B) = P(B \vert I)$ is the post-argument belief in 𝐵.
  • Informative: The assertion of the premise is informative (the argument is informative) iff $P_i(B) > P(B)$

  • Informativeness: The informativeness of the argument is $P_i(B) - P(B)$

  • Persuasive: The argument is persuasive iff $P_i(A) > P(A)$

    • Alternatively, the argument is persuasive if the argument is relevant and informative
  • Persuasiveness: The persuasiveness of the argument is $P_i(A) - P(A)$

Key Equations

And here is a summary of key equations in this series: $\label{1}$

  • Jeffrey’s Rule: $$P’(A) = P(A \vert \bar{B}) + P’(B)R(A,B)\tag{1} $$
  • Relevance of Rejection of Premise/Conclusion: $R(A,B) = -R(A,\bar{B}) = -R(\bar{A},B) = R(\bar{A},\bar{B})$
  • Symmetry of Corelevance: $CR(A;B,C) = CR(A;C,B)$
  • Necessity = Relevance × Acceptance: $N(A,B) = P(A) - P(A \vert \bar{B}) = R(A,B)P(B)$
  • Sufficiency = Relevance × Rejection: $S(A,B) = P(A \vert B) - P(A) = R(A,B)P(\bar{B})$
  • Relevance = Necessity + Sufficiency: $R(A,B) = N(A,B) + S(A,B)$
  • Sufficiency/Necessity of Rejection of Premise/Conclusion: $N(A,B) = S(\bar{A},\bar{B})$ and $S(A,B) = N(\bar{A},\bar{B})$
  • Persuasiveness = Relevance × Informativeness: $ P_i(A) - P(A) = (P_i(B) - P(B))R(A,B) $

Numerical Example

The following example illustrates all of the concepts introduced in this series.

Suppose the priors of the subject are modeled by the probability measure 𝑃 given in this table:


The marginal probabilities are:

$$ \begin{aligned} P(A) &= P(A,B) + P(A,\bar{B}) = .40 + .25 = .65 \cr P(B) &= P(A,B) + P(\bar{A},B) = .40 + .10 = .50 \end{aligned} $$

And the conditional probabilities:

$$ \begin{aligned} P(A|B) &= \frac{P(A,B)}{P(B)} = \frac{.4}{.5} = .8 \cr P(A|\bar{B}) &= \frac{P(A,\bar{B})}{P(\bar{B})} = \frac{.25}{(1 - .5)} = .5 \end{aligned} $$


Which lets us calculate the relevance:

$$ R(A,B) = P(A|B) - P(A|\bar{B}) = .8 - .5 = .3 $$

Necessity and Sufficiency

The necessity of 𝐵 to 𝐴 is:

$$ N(A,B) = P(A) - P(A|\bar{B}) = .65 - .5 = .15 $$

And the sufficiency of 𝐵 to 𝐴 is:

$$ S(A,B) = P(A|B) - P(A) = .8 - .65 = .15 $$

Notice that relevance is the sum of necessity and sufficiency:

$$ R(A,B) = N(A,B) + S(A,B) = .15 + .15 = .3 $$

And that necessity is relevance times acceptance:

$$ N(A,B) = R(A,B)P(B) = .3 \times .5 = .15 $$

And that sufficiency is relevance times rejection:

$$ N(A,B) = R(A,B)(1 - P(B)) = .3 \times (1 - .5) = .15 $$

Post-Argument Belief

Now suppose the assertion of 𝐵 in support of 𝐴 causes the subject to increase their belief in 𝐵 from $P(B)=50\%$ to $P_i(B)=90\%$.

The subject’s post-argument belief in 𝐴 will be, according to formula $\eqref{1}$:

$$ \begin{aligned} P_i(A) &= P(A|\bar{B}) + P_i(B)R(A,B) \cr &= .5 + .9 \times .3 \cr &= .77 \end{aligned} $$

This is slightly less than $P(A \vert B)=.8$ because the subject still harbors some doubt about 𝐵.


The informativeness is:

$$ P_i(B) - P(B) = 0.9 - 0.5 = 0.4 $$


And the persuasiveness is:

$$ P_i(A) - P(A) = 0.77 - 0.65 = 0.12 $$

Notice that persuasiveness is equal to relevance times informativeness:

$$ P_i(A) - P(A) = R(A,B)(P_i(B) - P(B)) = 0.3 × (0.9 - 0.5) = 0.12 $$

Post-Argument Necessity and Sufficiency

If after the argument the subject were to learn additional information causing them to reject 𝐵, the new posterior would be $P_i(A \vert \bar{B}) = P(A \vert \bar{B}) = .5$.

The post-argument necessity is therefore:

$$ N_i(A,B) = P_i(A) - P_i(A | \bar{B}) = .77 - .5 = .27 $$

And if the subject were to learn additional information causing them to accept $B$ completely, then new posterior would be $P_j(A) = P_i(A \vert B) = P(A \vert B) = .8$.

The post-argument sufficiency is therefore:

$$ S_i(A,B) = P_i(A \vert B) - P_i(A) = .8 - .77 = .03 $$

  1. Oaksford, M., & Hahn, U. (2013). Why are we convinced by the ad hominem argument?: Bayesian source reliability and pragma-dialectical discussion rules. In F. Zenker (Ed.), Bayesian argumentation: The practical side of probability (pp. 39–58). Springer Science + Business Media. https://doi.org/10.1007/978-94-007-5357-0_3 ↩︎

  2. Hahn, U., & Oaksford, M. (2007). The rationality of informal argumentation: A Bayesian approach to reasoning fallacies. (https://psycnet.apa.org/record/2007-10421-007) Psychological Review, 114(3), 704–732. https://doi.org/10.1037/0033-295X.114.3.704 ↩︎

  3. Hahn, U., Oaksford, M., & Harris, A. J. L. (2013). Testimony and argument: A Bayesian perspective. (https://psycnet.apa.org/record/2013-00206-002). In F. Zenker (Ed.), Bayesian argumentation: The practical side of probability (pp. 15–38). Springer Science + Business Media. https://doi.org/10.1007/978-94-007-5357-0_2 ↩︎

Next in This Series

Relevance and Corelevance
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