Complex sentence: 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.
Split into simpler sentences: It avoids Parfit's repugnant conclusion, but forbids what Parfit calls "mere addition". He argues that addition of extra, worthwhile lives which do not affect anyone else cannot make the outcome worse.