How does the hitahita chapter argue that no substance is wholesome or unwholesome in every respect?

It rejects the one-sided view that water, ghee, milk and rice are absolutely wholesome and fire, alkali and poison absolutely unwholesome: if the former were wholesome everywhere and always, people with new fever, diarrhoea, kushtha, fistula-in-ano, eye disease or wounds would have to take them constantly, while alkali, fire, instruments and poison used by skilled physicians relieve diseases at once.