Epistemic trespassers have competence in one field but pass judgment on matters in other fields where they lack competence. I examine philosophical questions related to epistemic trespassing by expert witnesses in courtroom trials and argue for the following positions. Expert witnesses are required to avoid epistemic trespassing. When testifying as an expert witness, merely qualifying one’s statements to indicate that one is not speaking as an expert is insufficient to avoid epistemic trespassing. Judges, litigators, and jurors can often recognize epistemic trespassing by examining a purported expert’s credentials and track record. Judges should not permit recognizable epistemic trespassers to testify as expert witnesses. Litigators should seek to expose recognizable epistemic trespassers during cross-examination. Jurors should treat recognizable instances of epistemic trespassing as a reason to downgrade the testimony of epistemic trespassers.
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