To be in good HEALTH, to be sound — the ordinary medical word for wellness (it gives English 'hygiene'), used literally of a healed body ('the healthy have no need of a physician', Luke 5:31). ⚠ The PASTORAL letters turn it into their signature metaphor for right doctrine: 'the SOUND teaching' (1 Timothy 1:10), 'sound words' (1 Timothy 6:3; 2 Timothy 1:13), teachers 'sound in faith' (Titus 1:13; 2:2). Doctrine is health; error is disease — and the image is carried to its grim end at 2 Timothy 2:17, where false teaching 'spreads like gangrene'. The recurrence of this one medical picture is part of what makes the three letters sound like a single voice. → first discussed at 1 Timothy 1:10