When does the notes’ rule treat a person of unsound mind as incompetent to testify?

Section 155(2), as quoted in the notes, makes the functional effect of mental infirmity decisive. A diagnosis alone is not enough; the condition must prevent the witness from understanding questions and furnishing rational answers. **Key takeaway:** Assess the effect of the condition on testimony, not the label alone. **Glossary:** Mental infirmity: a mental condition that may affect a person’s ability to understand or respond.