This is from Mark Twain's "The Innocents Abroad":
They flocked to our poor human doctor this morning when the fame of what he had done to the sick child went abroad in the land, and they worshiped him with their eyes while they did not know as yet whether there was virtue in his simples or not.
What in the world does "simples" mean here?
Have you looked it up online? There is a definition in the Lexico online dictionary of "simple" which says that it is an archaic noun meaning
A medicinal herb, or a medicine made from one
There are other, herbalist, sites which are still using the term to mean herbal medicines, in particular ones derived from one plant as opposed to mixtures and compounds.
I'm sure that's what Twain meant.
In the medicinal sense, a simple was a medicine that was composed only one active ingredient, especially a herb or other natural substance.
while they did not know as yet whether there was virtue in his simples or not. = although they were unsure, as yet, if his simples were effective.
OED:
B.4.a. A medicinal preparation containing or consisting of a single active ingredient; the active ingredient of such a preparation, spec. a medicinal herb. Now historical or archaic.
1646 Sir T. Browne Pseudodoxia Epidemica i. vi From the knowledge of Simples she had a Receipt to make white hair black.
1955 G. Grigson Englishman's Flora 201 The taboo against the use of iron in gathering simples was ancient and widespread through Europe.
2012 W. D. Storl Herbal Lore Wise Women & Wortcunners i. 12 The simples were cooked in salt or alkali and then filtered before being administered to the patient along with incantations.
Based on the broader context, I believe this is the meaning of simple as an ignorant person or a commoner, meanings 25 and 28 here: https://www.dictionary.com/browse/simple The "simples" are the native people. They are being described as the doctor's simples as if they are his disciples, who have spread news of his presence - though the rest of the native people don't yet know if there is virtue in his simples, i.e. they don't know if his disciples are correct when they report how this doctor can heal the sick.
