Sage is a herb used by centaurs, along with Mallowsweet, to assist in their stargazing. They burnt the herb and observed the flames and fumes.
Behind the scenes
Salvia and "sage" are derived from the Latin salvere (to save), referring to the healing properties long attributed to the various Salvia species. Modern evidence shows possible uses as an antisweating agent, antibiotic, antifungal, astringent, antispasmodic, estrogenic, hypoglycemic, and tonic.
Sage is also a known herb used in witchcraft and Paganism to banish negative spirits.