What did "simples" mean in the 1800s?

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?

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.

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.