2.3 The Acceptability of Reasons: Including Their Credibility

An argument succeeds only if (i) its reasons are acceptable, and (ii) they support the conclusion (Unit 3). This section is the test of (i).Grounds of acceptability (the four sources of a good reason):1. Observation — direct evidence ("I saw the collision"). Limits: perception is selective; lighting, distance, stress, expectation distort; memory is reconstructive, not playback (the psychology every cross-examiner exploits).2. Testimony — another's report. Strength depends on the witness's opportunity (could they know?), reliability (track record, character), and objectivity (stake in the matter).3. Authority/expertise — the expert's opinion. Legitimate when: genuine expertise in this field, within the consensus or properly reasoned, and honestly stated. Invalid appeal to authority: a film star on vaccines; a physicist on law.4. Prior knowledge / background information — well-established facts ("water boils at lower altitude temperature in Kathmandu").Credibility assessment (the checklist — apply to any source, print or person):• Vested interest — who gains if believed? (The interested witness doctrine; the interested publisher.)• Bias — ideological, financial, institutional; note that bias does not falsify — it lowers prior weight and demands corroboration.• Reputation & track record — the paper that has had to retract; the witness who has twice changed the story.• Corroboration — independent sources converging (one source is testimony, converging sources are evidence — History's rule, and the court's).• Competence & proximity…