It avoids Parfit's repugnant conclusion, but forbids what Parfit calls "mere addition", arguing that addition of extra, worthwhile lives which do not affect anyone else cannot make the outcome worse.
It avoids Parfit's repugnant. This Parfit's repugnant is conclusion "mere addition". Addition of extra, worthwhile lives cannot make the outcome worse. This is what it is arguing. These extra, worthwhile lives do not affect anyone else. It forbids what Parfit calls "mere addition", arguing that addition of extra, worthwhile lives cannot make the outcome worse. These extra, worthwhile lives do not affect anyone else.