Courtroom (snoring witness)

Bob testifies that he saw the defendant at the scene. Alice argues Bob was asleep at the time and could not have seen anything.

A compact illustration of undercutting — attacking a premise rather than a conclusion. Adapted from Modgil & Prakken's ASPIC+ tutorial.

The argument structure

Alice's argument undercuts Bob's witness-position premise (not Bob's conclusion). If Bob was asleep, his testimony provides no evidence about the defendant one way or the other. This is different from rebutting Bob — Alice is not claiming the defendant wasn't at the scene; she is claiming Bob's evidence is void.

Courtroom — undercut

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Alice's undercut (dashed) attacks the premise P, not the conclusion C. If U is accepted, P is no longer available, and the support P → C silently drops out.

The scheme

Argument from witness testimony

Premises

• W was in position to observe E.
• W asserts having observed E.

Conclusion

∴ E happened.

Alice's challenge keys on the first critical question. If the framework accepts "Bob was asleep at the time" as credulously accepted, the premise-attacker wins and Bob's testimony is undercut.

Rebut vs undercut, at a glance

• Rebut: attacker's conclusion contradicts target's conclusion. Both are about the same claim.
• Undercut: attacker's conclusion denies a premise or inference step the target relied on. The target's conclusion is not directly contested, but its support collapses.

In Dung's abstract framework, both reduce to "attack" — the distinction matters at the structured-argumentation layer (ASPIC+).

Further reading

• Bex, Prakken, Reed, Walton (2003) — argumentation in legal/forensic reasoning.
• Modgil & Prakken (2014) — undercutting in ASPIC+.