Following a chain of clues

Return to the factory machine from earlier. An equipment specialist has learned that several clues together can point toward a worn bearing.

The rulebook might contain this chain:

IF vibration is strongest near the spinning rod
AND the sound grows when the machine works harder
THEN suspect a worn bearing

IF a worn bearing is suspected
AND the outer shell is hotter on one side
THEN recommend inspecting the bearing

The first rule produces a new clue: a worn bearing is suspected. The second rule can then use that clue.

The inference engine follows the chain from observations to a recommendation. The specialist supplied the judgment behind each step.